Maybe one of the hidden APs and/or routers have the same SSID

Just changed mine. Maybe it’s disconnecting, and trying to connect to the wrong one.

I’ll probably replace the AP eventually.

EnGenius looks the same, and has controller software. Does it let you fully disable WPS or not support WPS? The iPhone can’t tell me if something has WPS enabled or not. If I get a Pi 3 B+, I can use that to find out.

It has a web interface. But there might be a cheaper alternative.