[Bug]: Amoro optimization can result in the input files and the merged output files having the same number of files, and this can cause the merge to fail and keep triggering the merge task. #3855

Author: wardli

amoro-format-iceberg/src/main/java/org/apache/amoro/optimizing/plan/AbstractPartitionPlan.java

@@ -263,7 +263,9 @@ public Weight getWeight() {
   /**
    * When splitTask has only one undersized segment file, it needs to be triggered again to
    * determine whether to rewrite pos. If needed, add it to rewritePosDataFiles and bin-packing
-   * together, else reserved delete files.
+   * together. If it doesn't need to rewrite pos delete, add it to rewriteDataFiles so it can be
+   * merged with fragment files or other files during the second bin-packing, which prevents file
+   * loss and ensures proper file consolidation.
    */
   protected void disposeUndersizedSegmentFile(SplitTask splitTask) {
     Optional<DataFile> dataFile = splitTask.getRewriteDataFiles().stream().findFirst();
@@ -273,7 +275,10 @@ protected void disposeUndersizedSegmentFile(SplitTask splitTask) {
       if (evaluator().segmentShouldRewritePos(rewriteDataFile, deletes)) {
         rewritePosDataFiles.put(rewriteDataFile, deletes);
       } else {
-        reservedDeleteFiles(deletes);
+        // Add to rewriteDataFiles so it can be merged with other files (fragment files, etc.)
+        // during the second bin-packing. This prevents file loss and ensures files can be
+        // consolidated when possible.
+        rewriteDataFiles.put(rewriteDataFile, deletes);
       }
     }
   }