The latest update is that this issue appears to have become rare. It’s happened a couple of times but only for brief periods.
I can only surmise at this point that there is something which is affected by the rates settings but not tied to it?
I can only leave it as-is and keep updating this thread if the behaviour changes.
Another update, 8 days since the last post.
No detected issues during this time.
I have now moved all APs back to being under CAPsMAN management. No issues detected in the following 24 hours.
I’m going to call this one soon with a “solved” status about this being related to the custom rates.
Folks I’m going to call this one resolved. The solution was to not alter the default rates, either in CAPsMAN or if configuring the APs directly.