Lens shadow

I have a tokina 12-24 which does the same with the built in camera flash. It goes away with the off camera flash pointing straight up so it bounces off the ceiling. Also, you might want to try leaving the lens hood off as well.

turn the on camera flash way down or redirect it. or better yet. get a set of cheap radio triggers and don't use it.

set you umbrella up high angle slightly down and you get lower so they fire just over your head. or stand behind them and so just the lens stick past them.