Difficult to reproduce problems while running tests

I have also noticed this problem (with 8.3.28) and with earlier version 8.3.7, as I reported here:

Ann: SWI-Prolog 8.3.7 - #5 by swi.

In between 8.3.7 and 8.3.28 I did not notice the problem.

This one is hard to reproduce, as it often is with thread deadlock problems.