Hello
This bug should be already fixed in stable branch and will be shipped in next minor release. Until the fix is released, the simplest workaround is to disable parallel query with SET max_parallel_workers_per_gather = 0 if you're affected by the bug.
It's a rare timing bug, but some workloads seem to be more prone to it, so unfortunately it went undetected for a long time.
If you're interested, related discussion and patch was here: https://www.postgresql.org/message-id/flat/20190207014719.GJ29720%40telsasoft.com
regards, Sergei