I can reliably state that this issue happens while the device is stationary and/or roaming. I've seen it in both instances. It seems to predominantly affect iphones and ipads, but I have seen it (very rarely) on my Mac. I'm guessing I've ran into it on my HomePods as well, but those have hardly any interface at all, so that's nothing more than a guess.
I wish I could identify other patterns, but even after months of observation, I still can't detect any. Lily & musubi are the two most-used APs in our household, with lily doing the lion's share of that work: think of our house as a long rectangle, and lily is in the center, while musubi and gus are towards either end.
AFAIK performance is flawless before the drop; definitely not a "slow and steady decline." The drops happen regardless of usage, as I've seen them in the morning when I wake up and find my wife's phone offline while mine is online, other times I'll be in the middle of watching a YouTube video and it'll drop half-way through. The drops happen per-device—one device will go offline while the others in my household are unaffected.
Drops can happen as frequently as 3x per day, or as infrequently as 3x per week. Also the frequency is not tied to the device/OS type, as my iPad may drop 3 times yesterday, then it won't drop again until 2 days later.
Lastly, just wanted to repeat my original post to say that there are two different types of drops with two different recovery mechanisms. Those also do not appear to be device/OS specific, and there is no pattern (that I can detect) about when I'll encounter one type or another.
The infrequent, seemingly non-reproducible nature of the problem is why I waited so long to post about it here on this forum, as I was afraid I'd get a lot of "huh, weird. I dunno. 
" since I was coming in without reliable reproduction steps.