The percentage of deleted docs needs to be strictly over 10% for deleted docs to be expunged.

core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -1371,7 +1371,7 @@ public class IndexShardTests extends IndexShardTestCase {
             final long numDocs = randomIntBetween(2, 32); // at least two documents so we have docs to delete
             // Delete at least numDocs/10 documents otherwise the number of deleted docs will be below 10%
             // and forceMerge will refuse to expunge deletes
-            final long numDocsToDelete = randomIntBetween((int) Math.ceil(numDocs / 10.0), Math.toIntExact(numDocs));
+            final long numDocsToDelete = randomIntBetween((int) Math.ceil(Math.nextUp(numDocs / 10.0)), Math.toIntExact(numDocs));
             for (int i = 0; i < numDocs; i++) {
                 final String id = Integer.toString(i);
                 final ParsedDocument doc =