It´s flat on top because I took a small pause before operating the yaw stick back. Betaflight is almost identical to cleanflight. I´ve used them bothIn that video where you operate the yaw. That should be a nice smooth sinusoidal waveform. There is definite clipping going on where the peaks of the wave are cut off flat until you drop below a certain level where it begins to follow the sticks again. I am guessing that is where it takes off on you in flight.
Once again I am not familiar with Beta flight I have only run clean flight before and never messed with filters. In my Kiss set up they are turned totally off as from what I gather they are to smooth out a noisy system and are only used AFTER a problem is found and needs hiding to clean up the signal.
At this point if no one here has a specific answer I would send an email to the programmer thru their support and link that video along with screenies of your pids and your filters settings.