If it's firmware, does it work to downgrade back to the version that didn't exhibit this? (assuming it's possible to do this; I haven't tried). Older firmware should be available from Grin (if it's not on the website you could see if they have a version they can send you). I looked in my folders and found these, if they're helpful
View attachment 324930
View attachment 324931
View attachment 324932
View attachment 324933
Also, there is a hardware issue with some older units, bad capacitors somewhere IIRC. You can contact Grin and see if yours is an affected unit.
On mine, I have had the fluctuating voltage and power fault when I use it on a battery with a BMS that isn't turning on the charge port (or turns it on then back off for whatever reason).