some vfd has this

I remember my first spindle made it..
and I don't even could set to 400 hz

best you can do, use it as long as you can, and change it when you get chance.. I had a sunfair c300 vfd.. if it helps..
I wrote email to manufacturer , they answered.. but fluctuating were remain..

changing both, spindle and vfd I didn't experienced anymore..

I look around on other computers, because I remember the manual was discussing the fluctuation.. with settings you can reduce it..