Around Christmas time I moved the Home Assistant controller onto a VM on my home server, rather than being a Pi 4. There was some odd instances of not being able to connect to it, usually from our mobiles, that I couldn't quite put my finger on.
And sometimes the mobiles wouldn't hop on to the home wifi, but sit on mobile data complaining about a bad password. I didn't think these were related.
Anyway, I started looking at the living room access point (a Mikrotik thing, so very powerful but relatively easy to mess up), to check the configuration and passwords to figure out what was going on with the mobiles. Turns out when I put that in, I'd made a typo in the IP address and put it into a range I use for server stuff, rather than the networking stuff. Which was fine, until I'd put the new instance of Home Assistant on to the server, and gave it the next "free" IP, which happened to be the same as the one being used by the access point. Hence weird issues, depending upon which of the three access points in the house the mobile device connected to.