Can no longer share Keynote to external A/V since Sonoma

For years I've run Keynote presentations in venues all over the world. Never had a problem – until I upgraded to Sonoma.

Before Sonoma, I'd plug into the venue's A/V system via HDMI and was able to project my Current Slide onto the public screens while seeing the Presenter Screen (Current Slide, Next Slide, Notes, Timer, etc) on my MacBook. Sometimes I had to fiddle around with the settings in Displays to make sure Mirroring was off. But it ALWAYS worked.

And then came Sonoma....

When I plug into an external A/V system now, I cannot get it to share my Current Slide on the public screen while showing the Presenter Screen on my laptop. I've tried every possible configuration in Displays and ... nothing.

Either the same thing appears on my MacBook and the external screen. Or the Current Slide or Presenter Screen appear on the external screen while my laptop screen remains black.

Sometimes just plugging into an external network wrests control away from my MacBook screen. The arrow and the open Displays window appear on the external screen while my MacBook screen greys out. (I can usually drag the arrow back across to the laptop screen.)

And there are a few other weird permutations that occur....

I have now run into this problem at three events in three different countries. Each time the seasoned tech team were stumped and could not make it work. So we moved my Keynote presentation onto one of their MacBooks (running Ventura) and it worked perfectly in every case, every time. Plug and play, like it used to be on my MacBook.

I've spent hours on the phone with the Apple Helpline and spoken to two different experts at the Genius Bar in London. No one could understand or fix the problem. The closest I got to an explanation was one of the 'geniuses' complaining that Sonoma was causing lots of software problems.

Bottom line: I can no longer use my MacBook to run Keynote presentations on an external network.

Does anyone know what's going wrong here? Or how to fix it? Has anyone else had a similar problem? Has Apple said anything useful it?

I'm getting to the point where I'm thinking of wiping my MacBook and reverting to Ventura – which is ridiculous.

Any help gratefully received!!

Carl

My MacBook is an Apple M1 Max with 64GB of RAM. And I'm running Sonoma 14.1.1.