I had a similar issue and it related to the uplink connectivity monitor. Unifi monitors the gateway and controller IP and withdraws the SSID if it can’t contact either of them. Make sure you have turned off wireless meshing on the device and I think you can manually disable the uplink connectivity monitor.
I used to work with this idiot. Full of platitudes and shite. He’d come into the office in the morning and ask “good morning, how are you?”. Even if you said “my cat got run over”, he’d say “that’s great” and carry on his day.