Hmmm, this sounds familiar!
I didn't realize it either. Not sure anybody knew as flips with self level on are generally not done. It's only the recent release of 1.6++ that has brought this to light.
I'm no flip expert... but this is my take on what you're supposed to do for a stationary flip (ie not traveling in any particular direction):
- power up so that you're moving vertically upwards.
- reduce throttle (DO NOT CUT) and apply aileron
- keep aileron applied until quad is past inverted!!
- apply lots of throttle to recover!
Please be aware that the above is only my interpretation of various posts on RCG and from watching a few video's. My quad is (again) in the same state as your 450.