mira.frame not adjustable on Mac

I open a mira.frame on a Mac Studio Ultra and it appears for 2 seconds in the regular way, only to change into a grey rectangle with the green square on the left top, but unadjustable, cannot move it nor change the size. Clicking on it does not activate it. No inspector acres. But I can move objects onto it which then appear on the iPad but much to large because the frame has the start size, which is small.
Seems like a bug, because it is repeatable, always the same.