Hey all, I see there's a few different issues in this thread, wanted to throw out there that I had a similar set of symptoms on an F5-221 running 3x drives in BTRFS TRAID on TOS 5.1.24. I documented the fix for my issue in another thread, having to do with disabling quotas, which then allowed me to update to 5.1.33 where a general fix has seemingly been implemented.
viewtopic.php?f=71&t=4336
Totally possible that this isn't the same problem, but wanted to throw it out there in case it helps.