Via Polychop Discord (amongst other developer changelogs):
MMS diagonal slewing improvements for both axis and button inputs
No current ETA, just TBD - but I'd assume next patch. Hang on until then and it should hopefully be addressed.
In the meantime, I'll occasionally find the MMS gets a bit wonky after sustained use, nearly every time if i cycle the power to OFF, back to FWD, re-slave and start using it again, it'll behave. Hope this helps.