The fix for JDK-8221696 added MappedByteBuffer(int index, int length) to allow specifying a range to write back to storage. The no-arg MappedByteBuffer at this time and before wrote back the entire buffer content, i.e., the half-open range [0,capacity()). The new force(int,int) checked its parameters using Objects.checkFromIndexSize(index, length, limit()), i.e., with the limit instead of the capacity as upper bound. In JDK-8243491 the no-arg force() was changed as

@@ -293,8 +227,7 @@ public abstract class MappedByteBuffer
             return force(0, limit());
         }
         if ((address != 0) && (capacity() != 0)) {
-            long offset = mappingOffset();
-            force0(fd, mappingAddress(offset), mappingLength(offset));
+            return force(0, capacity());
         }
         return this;
     }

to call the new force(int,int). This inadvertently changed the upper bound of the no-arg force() from capacity() to limit() thereby provoking JDK-8246729. The upper bound of force() should be the capacity.