diff --git a/fesod-sheet/src/main/java/org/apache/fesod/sheet/write/metadata/style/WriteCellStyle.java b/fesod-sheet/src/main/java/org/apache/fesod/sheet/write/metadata/style/WriteCellStyle.java
index 2dc52ba7e..31d1683e6 100644
--- a/fesod-sheet/src/main/java/org/apache/fesod/sheet/write/metadata/style/WriteCellStyle.java
+++ b/fesod-sheet/src/main/java/org/apache/fesod/sheet/write/metadata/style/WriteCellStyle.java
@@ -179,7 +179,7 @@ public static void merge(WriteCellStyle source, WriteCellStyle target) {
         }
         if (source.getDataFormatData() != null) {
             if (target.getDataFormatData() == null) {
-                target.setDataFormatData(source.getDataFormatData());
+                target.setDataFormatData(source.getDataFormatData().clone());
             } else {
                 DataFormatData.merge(source.getDataFormatData(), target.getDataFormatData());
             }
diff --git a/fesod-sheet/src/test/java/org/apache/fesod/sheet/style/ContentStyleDataFormatPollutionTest.java b/fesod-sheet/src/test/java/org/apache/fesod/sheet/style/ContentStyleDataFormatPollutionTest.java
new file mode 100644
index 000000000..7b3e90ed2
--- /dev/null
+++ b/fesod-sheet/src/test/java/org/apache/fesod/sheet/style/ContentStyleDataFormatPollutionTest.java
@@ -0,0 +1,147 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.fesod.sheet.style;
+
+import java.io.File;
+import java.util.Arrays;
+import java.util.List;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import org.apache.fesod.sheet.FesodSheet;
+import org.apache.fesod.sheet.annotation.ExcelProperty;
+import org.apache.fesod.sheet.annotation.write.style.ContentStyle;
+import org.apache.fesod.sheet.enums.CacheLocationEnum;
+import org.apache.fesod.sheet.metadata.data.DataFormatData;
+import org.apache.fesod.sheet.testkit.assertions.ExcelAssertions;
+import org.apache.fesod.sheet.testkit.base.AbstractExcelTest;
+import org.apache.fesod.sheet.testkit.enums.ExcelFormat;
+import org.apache.fesod.sheet.write.metadata.style.WriteCellStyle;
+import org.apache.fesod.sheet.write.style.HorizontalCellStyleStrategy;
+import org.junit.jupiter.api.Test;
+
+/**
+ * A registered cell style strategy must not overwrite the cached {@link ContentStyle#dataFormat()}
+ * of a field. Before the fix, {@code WriteCellStyle.merge} aliased the annotation's shared
+ * {@link DataFormatData} into the per-cell style without cloning, so a later merge from another
+ * style source mutated the cached annotation format in place and subsequent cells (or, with
+ * {@link CacheLocationEnum#MEMORY}, subsequent writes) silently lost the annotation's number format.
+ */
+public class ContentStyleDataFormatPollutionTest extends AbstractExcelTest {
+
+    private static final short STRATEGY_FORMAT = 2; // "0.00"
+    private static final String ANNOTATION_FORMAT = "#,##0.00"; // builtin index 4
+
+    @Data
+    @NoArgsConstructor
+    @AllArgsConstructor
+    public static class AnnotationFormatData {
+        @ExcelProperty("value")
+        @ContentStyle(dataFormat = 4)
+        private Double value;
+    }
+
+    /** Separate bean class so the static MEMORY cache of the other test cannot interfere. */
+    @Data
+    @NoArgsConstructor
+    @AllArgsConstructor
+    public static class AnnotationFormatMemoryData {
+        @ExcelProperty("value")
+        @ContentStyle(dataFormat = 4)
+        private Double value;
+    }
+
+    /**
+     * Content style list alternating between a style that defines a data format and one that does
+     * not, so odd data rows must fall back to the annotation's format.
+     */
+    private static HorizontalCellStyleStrategy alternatingStrategy() {
+        WriteCellStyle withFormat = new WriteCellStyle();
+        DataFormatData dataFormatData = new DataFormatData();
+        dataFormatData.setIndex(STRATEGY_FORMAT);
+        withFormat.setDataFormatData(dataFormatData);
+        WriteCellStyle withoutFormat = new WriteCellStyle();
+        return new HorizontalCellStyleStrategy(null, Arrays.asList(withFormat, withoutFormat));
+    }
+
+    @Test
+    void annotationFormatSurvivesStyleStrategyInSameWrite() throws Exception {
+        File file = createTempFile("dataFormatPollution", ExcelFormat.XLSX);
+        List<AnnotationFormatData> rows = Arrays.asList(
+                new AnnotationFormatData(1111.5), new AnnotationFormatData(2222.5), new AnnotationFormatData(3333.5));
+
+        FesodSheet.write(file, AnnotationFormatData.class)
+                .registerWriteHandler(alternatingStrategy())
+                .sheet()
+                .doWrite(rows);
+
+        try (ExcelAssertions ea = ExcelAssertions.assertThat(file)) {
+            ea.sheet(0)
+                    .row(1)
+                    .cell(0)
+                    .hasDataFormat(STRATEGY_FORMAT)
+                    .and()
+                    .and()
+                    // The strategy style for this row defines no format, so the
+                    // @ContentStyle(dataFormat = 4) annotation must win.
+                    .row(2)
+                    .cell(0)
+                    .hasDataFormatString(ANNOTATION_FORMAT)
+                    .and()
+                    .and()
+                    .row(3)
+                    .cell(0)
+                    .hasDataFormat(STRATEGY_FORMAT);
+        }
+    }
+
+    @Test
+    void annotationFormatSurvivesAcrossWritesWithMemoryCache() throws Exception {
+        List<AnnotationFormatMemoryData> rows =
+                Arrays.asList(new AnnotationFormatMemoryData(1111.5), new AnnotationFormatMemoryData(2222.5));
+
+        File first = createTempFile("dataFormatPollutionMemA", ExcelFormat.XLSX);
+        FesodSheet.write(first, AnnotationFormatMemoryData.class)
+                .filedCacheLocation(CacheLocationEnum.MEMORY)
+                .registerWriteHandler(alternatingStrategy())
+                .sheet()
+                .doWrite(rows);
+
+        // A completely separate write without any custom handler: every data row
+        // must show the annotation's format.
+        File second = createTempFile("dataFormatPollutionMemB", ExcelFormat.XLSX);
+        FesodSheet.write(second, AnnotationFormatMemoryData.class)
+                .filedCacheLocation(CacheLocationEnum.MEMORY)
+                .sheet()
+                .doWrite(rows);
+
+        try (ExcelAssertions ea = ExcelAssertions.assertThat(second)) {
+            ea.sheet(0)
+                    .row(1)
+                    .cell(0)
+                    .hasDataFormatString(ANNOTATION_FORMAT)
+                    .and()
+                    .and()
+                    .row(2)
+                    .cell(0)
+                    .hasDataFormatString(ANNOTATION_FORMAT);
+        }
+    }
+}