I agree, I also suspect it is a bug. I can see that if the project is in SI, the HT curves for the bridge are generated with wrong elevation values. The values of all curves I got were under the terrain and so practically not allowing water to flow over the terrain, under the bridge. I did not find a way to manually edit the HT curves, so sent a report about it to HEC.
An interesting test if someone has time for that would be to convert the project (and not just switch to) to US units, securing correct HT-curve elevations, and converting back after the simulation.