Hi,
I have been having the same issue and I think it is a bug.
I’ve tried countless iterations (including SI and US units) to try and get it working but nothing helps. Interestingly, I started to create bridges in the existing example project of Bald Eagle Creek. It worked fine in there provided I was using the existing terrain within the model. I could delete everything (except the terrain), create a new 2d flow are, new boundary conditions and new bridges and they worked fine. Once I deleted the terrain however and replaced it with something else (or even the same terrain added back in) any structures begun to act like a dam again.
Hoping they are able to fix this soon as this function would be very useful for a current project! Sorry I couldn’t be any more help.