Okay, here's a hotfix for Quick Turn, just change Max to Min and Min to Max:
This could be happening because my Walk XSpeedMax speed (40) is lower than my Decel speed (77), but I also haven't noticed any difference with Quick Turn on or off, perhaps because of the speed being different.
EDIT: this actually doesn't work since it breaks if you hold left and right down simultaneously! I think the quick speed could be quicker if it didn't have to reapply acceleration after the turn, I'm not sure where this could be set though (since RPM seems to work out the acceleration amount based on the turn somewhere else, Quick Turn current seems to just reduce deceleration immediately.
EDIT #2: with Quick Turn off, the player just keeps walking in the same direction, I'm not sure if that's intentional?