This one is a tricky one, i don't think it's a bug, more a clash betwenn two tools?
The union of two simple squares works fine. The way they were united have the consequence, that some points are two times in the main path.
It means that when editing the options of the detail piece, nothing can be saved because there is the error "you have double points!" in the main path.
If we consider a union to be a specific case, maybe the main path shouldn't be editible in this case and shouldn't be checked?
In the uploaded val file, there are in detail mode the 2 original squares and the united piece.