Chemderama wrote:
I’m having exactly the same issues (also need to be wireless due to teaching). I find that if I unplug the hdmi to projector it will then mirror properly and then I can plug the hdmi back in. It is very unreliable though. Very disappointing update.
Thank you, I think that your observation - that disconnecting the projector helps - sheds some light on the issue.
FWIW, this seems to be something that occurred in Sequoia as well: in some cases, although connecting via HDMI, the Mac was "Airplaying" to the projector, instead of using it as a normal, directly attached, external display.
The obvious efffect was that it caused lower display quality on the projector, but in the present situation this also prevents another device from being used with (real) Airplay.
I do use projectors for teaching and I saw this happen a few times (but I do not use mirroring to an iPad).
There were several reports of this with HDMI projectors (and tv also).
Here is one thread about it from Sequoia:
"Second" monitor set to AirPlay mirror ra… - Apple Community