Solved by placing two projection sets (an extra one just with the corner views with a different scale). Not comfortable, but works as expected.
This is maybe another one for the Master Of TechDraw wandererfan. (Thanks for all your efforts!)
Each item of a projection group has its own scale property.
Changing this does not change anything in the drawing.
Is it intended to work?
Or should the scale property of the item been hidden?
Same behaviour in Version 0.18.16131 and 0.19.18542
can be used for testing. Just try to shrink an item at the corners This feature is useful (in my opinion), but it might break ISO / ASME conformity.