src/java.base/share/classes/java/util/stream/StreamSpliterators.java

@@ -187,10 +187,12 @@
         @Override
         public Spliterator<P_OUT> trySplit() {
             if (isParallel && !finished) {
                 init();
 
+                if (buffer != null && buffer.count() > 0) // partial traversal started
+                    return null;
                 Spliterator<P_IN> split = spliterator.trySplit();
                 return (split == null) ? null : wrap(split);
             }
             else
                 return null;