In Detail mode, I used the union tool to unite 2 simple squares. They are then united, everything is fine.
When I perform the undo union tool, the 2 detail pieces can't be moved anymore with drag and drop. It only affects the pieces that were united. When saving, closing and reopening the pattern, everything is back to normal and the pieces can be moved again.
I uploaded a test val file, you just need to go to the Detail mode, the two squares are there for test.