When I check the Google Home app, I see my Sonos speaker pair in the correct room. A couple of times the problem fixed itself spontaneously after a while. A couple of times I was able to fix it manually by removing the Sonos speaker pair from the room and re-adding it. And a couple of times, like today, that didn't work because Google Home stubbornly says "There was a problem communicating with Google" whenever I try to touch the Sonos speaker room assignment (and it only affects Sonos, I can move other devices between rooms in GH without this message appearing).
Today I gave up and unlinked my Sonos account from Google and re-linked it. That worked as a last resort last week. Today it resulted in another disaster. After unlinking and re-linking my Sonos account, the speaker re-appeared in the correct room, but the microphone light went out and would not go on again even after unplugging and re-plugging power a couple of times. So I re-did the whole Google Assistant setup, and that got it back (had to re-do the settings from scratch). And it spontaneously re-appeared in the correct room, and now appears to know which room light to control again.
What a total mess! And given that it already happened 5 times this week, it's likely to happen again.

What's going on here between Google and Sonos?