Yes, so in the iPadOS 27 patch notes it says they've changed something with how pins work, so I'm thinking on the ViewSonic AirPlay settings, could you try turning off the pin as a test to see if it now works.
Just looking at what they've changed in Ipad os 27 are you using PINs or anything like that for AirPlay?
Maybe try turning off pins and see if that works.
An easier option might be to restore said iPads to 26.6 and block iPadOS 27 through your MDM for now.
*EDIT just found out that 26 is not signed anymore
Just remember: buy cheap, buy twice.
You might as well just buy the UniFi equipment and know what you're getting.
Nothing wrong with Yealink phones, though.