This is roughly as expected/designed -- we won't auto tag with a [ after it because in all likelihood, it's a BB code. It's really part of the challenge when using an embedded markup along with standard text parsing.
Is it possible to check whether it is a BB Code or not?
I guess the only challenge there is it may not be possible to know which BB Codes have been added via the load_class_bb_code event. The default ones we know, and the new custom ones are cached. Not sure of the feasibility of detecting whether a tag has been added via a load_class_bb_code event.