I’ve been seeing a very similar sounding behavior on my MK3 for months now, with the 2nd & 3rd SSID (WPA2 personal) basically shutting down 1-2 times per day. By shutting down, I mean often the problem SSIDs stop being broadcast & all devices on them can’t connect because there’s “no WiFi”. (i.e. They don’t become hidden - they really stop existing.) The 1st SSID seems to keep working fine (WPA2 Enterprise) even though the 2nd & 3rd have disappeared & disconnected the devices originally connected to them. I originally thought it had to do with some range extenders for the 2nd & 3rd SSIDs, but the range extenders were removed from the network months ago. I mentioned this in the forums a while back, but have been traveling too much to open a support ticket. My “work around” has been to put the radio (AP → Settings) on a schedule that shuts it off entirely twice a day. When the radios resume, everything reconnects and works for a while. The downside is the schedule (System → Schedule) is poorly implemented with a 30 minute granularity, so the WiFi is offline for 1 hour out of every 24. (The schedule feature is almost useless having on a 30 minute granularity - why not make it 1 minute?) In normal use, the 1st SSID has a few devices, 2nd SSID has 10 devices connected to it, and the 3rd SSID has 6. The 1st & 3rd SSID’s are on the untagged VLAN, the 2nd SSID is on a tagged VLAN. The system has a 3rd tagged VLAN that is Ethernet only. The main WAN connection is Ethernet (to Comcast), the 1st fallback is USB to an AT&T hotspot (Standby), the 2nd fallback is WiFi to a Comcast Xfinity hot spot broadcast from nearby (Cold Standby).
I wonder if using WiFi as WAN in any capacity screws up the long term WiFi as LAN behavior(?)