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.