I had previously tested upgrading a Remote Desktop installation from 2012R2 all the way to 2022 in a test sandbox environment, which all went well, so I planned to upgrade our production deployment.
My Plan was to go from 2012R2 to 2019, then 2019 to 2022, however, one connection broker failed to upgrade to 2019, with the error after rollback indicating some sort of Sysprep issue. The other, currently inactive connection broker (you're supposed to remove all but one broker in a HA deployment and then do the in place upgrade) upgraded to 2019 without issue, although it took FOREVER. I then tried to upgrade to 2016 instead of 2019, and while that also took forever, it succeeded, and once I had patched the 2016 connection broker to address the CredSSP issue, I was able to connect fine to several different collections.
When everyone started coming in, there were a lot of issues though - Some getting the message 'The requested session access is denied', which is usually a damaged RDP file that's causing you to try to be connected to the session broker instead of a deployment server. The other time it can come up is with macs, if you have the admin session box checked in the RDP app.
After a bit, it became apparent that only macs were having issues, which explained why I couldn't duplicate the issues even when connecting as users that were reporting issues.
I then saved a snapshot of the connection broker in it's 'upgraded to 2016' state and reverted back to the 2012r2 snapshot, after which, everyone, including mac users, could connect.
I'm not sure what else I can provide, but the TL;DR is I upgraded the active Remote Desktop Connection Broker from 2012R2 to 2016, and after that, macs could not connect to any session host in the deployment, but PCs could connect fine.
That being said, does anyone know if this is a known issue with 2016 or 2016+ and macs, was it resolved at some point, and/or where should I look to determine why the macs wouldn't work but PCs would?