Hi Jon,
1. No, I didn’t. I also restarted my computer to check for this and have a fresh launch of Premiere Pro.
2. I did see the yellow hazard icon and the message was related to a resolution change on the monitor, so it advised to restart PPro to avoid any issues, I did that and the issue remained.
3. Nope, I haven’t unplugged it and the connection is stable, I can use it with other software and it works as expected, is not mirroring.
The only modification I did in the past few days is attach a third monitor, which required a driver to work, due to the fact that it is connected via USB, not the usual mini port/Thunderbolt port on my iMac. I tried getting the external monitor to be detected with the third one both attached and not, but without success.
Thanks.