Yeah, my problem was almost identical to yours. On my iOS devices, iPhone and iPad, I would get occasional prompts to rejoin the WiFi and input the password for it. My watch is what really tipped me off, though. It wouldn’t connect to the WiFi at all. I went digging around the keychains using my Mac and my home WiFi was saved just to the system keychain, not both it and iCloud. So the Mac seems to work fine like that, my guess is that the iOS devices would periodically dump the home WiFi network since it never made its way to the iCloud Keychain. I deleted the WiFi networks from the system keychain as well as the iCloud Keychain on my Mac. I then rejoined the home network and saw that it properly synced over to iCloud and that problem has went away. Not sure if that will do the trick for you, but worth a shot.