I hit this issue when the processService.getProcessInstanceVariable from the Reoccurring command is accessing the same process instance that is at the time processing a REST signal.
However I was able to reproduce it only where there were two kie pods (no errors with one pod, didn't test with more).

Caused by: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect)

The issue is probably related to
https://github.com/kiegroup/jbpm/commit/6314dd2a4ef9bc4a27724d4724f86ceac2f1f04d