Ctrl-clic stone on tower : moved to invisible slot ?

I did a ctrl-clic to move a teleportation stone to tower with a teleportaion stone already in place (in order to replace it), instead the stone seem to ahve move to an invisible spot :
- It was impossible to start the tower with the stone in it (error message when pressing start)
- When I removed the visible stone, the button stayd available, and when il clicked it the tower starts with the hidden stone powers