In this case a bridge over the end of another bridge shows are bridge graphic as if the levels were different but everything is on the same level. It operates as a bridge on a different level. R7771 Win64.
A bridge should not be allowed there in the first place, as there is zero clearing below ...
It is indeed possible to build a flat bridge on top of other bridge heads (bug). To work around this, you can press 'ctrl'-key while building.
this should be fixed with r7785 now