I made it.
I don't know why. But I had to delete all checkboxes and create them new.
It only works in nested subforms if you create all checkboxes direktly in one step. If you format one and then create a new one it behaves like an option.
Could this be a bug?
How do you create / delete the checkboxes?
I did activate them in hierarchy and used keyboard key del.
Then drag and drop new ones from objects panel.
After placing them I formatted them.
The original file was created like this. Create one checkbox and copy it several times. Then renaming them.
I coul verify in another form that I never can copy a checkbox.
Still the same problems.