It looks like Airtime fairness has been changed.
Maybe that is the culprit?
I think it started happening earlier than that. Maybe when Vanilla firmware was updated (end of May), or even earlier when mac80211 tree was updated (beginning of May.) It's so inconsistent that it's hard to pinpoint.
For those having problems, do you have airtime fairness enabled?
Not in my case. You had suggested to disable it in early May, so I did and kept it that way since. The other possible setting raised there (Disassoc Low Ack) has also been disabled since that discussion.
In the end I rolled back to r48971 and will keep it that way. I only got the dmesg messages about failing to look up txq for peers a couple of times on that build, and we didn't actually notice the issue happening.. So I'll just stay on that build until either I need some specific new feature/fix, or I have enough free time to deal with possible issues.
TLDR for my case: The only difference between r48810 (May 6th build) and r49113 were the builds (48810, 48897, 48971 and finally 49113). I just don't have the means to deal with it, so I went back to the latest stable build for me.