You are correct. The execution of the hooks when creating a new topic was a misobservation on my side. Sorry about that!
The bbp_new_forum hook firing only when a forum is created outside wp-admin is interesting information. Thanks for clearing that up.
I have already worked around the ‘missing’ hooks by tying into the save_post hook. Its not exactly what i would call elegant, and it requires some additional checks to make sure my callback was executed for the right event, but it does the job.
So, the only thing that is left for me to ask is: Why isn’t there an intuitively named hook for when one creates a forum from the backend? Or is there one that i am missing? 🙂