Symptom: under certain conditions (to be determined, but our tests do not trigger the problem) the output file of a Snapshot is missing some entries.
The problem was first reported on the forum here. I can reproduce it locally, with a debug build of master as well as an optimized build of v6.14.04.
A first investigation points to lifetime issues of the thread-local TBufferMergerFiles that tasks write their task-local output TTrees to. Work in progress.