Participant Video panel disappears when screen sharing on Mac

I deliver Management training globally. I've just bought a new Macbook Pro which I run with an additional screen. As soon as I screen share PowerPoint, my participant video panel disappears, which is a major issue for me.

I've tried all side by side options, updating zoom etc., but nothing works.

It still works fine on my old Macbook. I've checked that the setting are the same on both machines. The only difference is the new machine is running OS Monteray 12.3, and the old is running Big Sur 11.6.2.

any ideas gratefully received!!