[operator]

Thanks everyone for your help. I found the bug.
It is between Pro Tools and the Mac Studio Hdmi output.
I have an Hdmi to Vga adaptor connected for one of my monitors. This was preventing the dropdown menu from displaying.
When the Hdmi output is connected directly to an Hdmi monitor the dropdown menu does display, however the "OK" at the bottom right of the Playback Engine window is absent, as are the 3 traffic light buttons at top left.. "Return" does close the window, but there is something wrong there.
Fwiw, Apple, Motu, iZotope and DaVinci have no problems with the Hdmi to Vga adapter.