I have noticed that the brush when drawing freehand, if smoothing is on, even a slight value, it like jumps or trembles while drawing.
you are stuck with this i'm afraid

xara has 'smoothing' but no stabilisation in the modern sense and there is no way round it - the program seems to try to guess where you want to go next which is back seat driving at its worst when sketching

it has been like this since 2003 when I first used the program and probably since inception; xara have never shown any inclination to change it - my honest advice: use affinity designer instead if this gets in your way [as it does for me]