I keep getting an under-voltage warning sporadically with my StellarMate Pi since I upgraded to the newest OS. I've tried it on different pi boards and it's the same thing so it's leading me to believe it's the OS flashed to the SD card. I've checked the hashes on it like the StellarMate website says but the Hashes are different than the expected Hashes. It seems to be working fine but the warning is a little concerning and I'm just wondering way it's doing it. I've also noticed that the noVNC viewer is a little slower after the upgrade. A few seconds of delay with every click. Is that normal?
Cant see why software update has caused this but only he Dev's will know for sure. Maybe someone will answer who knows.
Do you have a backup SD which was before the upgrade - if yes try that and see what happens!
However it could be your Power source is degrading for some reason showing a problem - plus longer or poor quality cables can also give the same problem. Have you another power source to try. Even if you haven't changed anything.
Normally you never see this via any VNC software (ones that I have used anyway) even if its shown on a monitor connected to the RPI. But the boot logs do show if this is happening.
AFAIK the error arises from the power supply - I was getting the messages but I have an Anker hub that has quick charge 1.5A port on it - the messages immediately disappeared when I plugged into the 1.5A plug on my RPi 3B+