I have finally fixed this problem, however it's worth noting that my flavour of the note9 is curtana (aka Indian note9), not joyeuse (aka Global note9).
Did this by "upgrading/downgrading" ROM: i.e. upgrading to eu.miui 12.0.6.0.QJWMIXM from 12.0.4.0.RJWMIXM, thus downgrading to Android 10.