1. "dsa_area could not attach to segment": dsm.c, fixed in commit 6c0fb941. 2. "cannot unpin a segment that is not pinned": dsm.c, fixed in commit 0b55aaac.
Yes, I found both entries in our logs, each once per incident.
The fix will be included in the next set of releases, but in the meantime you could consider turning off parallel query (set max_parallel_workers_per_gather = 0). In practice I think you could also avoid this problem by loading a library that calls something like srandom(getpid()) in _PG_init() (so it runs in every parallel worker making ID collisions extremely unlikely), but that's not really a serious recommendation since it requires writing C code.
This is very reassuring, as I was slowly starting to doubt my DBA competence here. :)
I'll try this workaround until the next release is out, will report if anything out of ordinary re-occurs.