I have my min/max RPM values ($30/$31) set in GRBL. I had to do this with the console as the Raspberry Pi build doesn’t appear to be updated (that’s fine).
It’s confusing how grbl handles this but I believe when you set $30=8000, essentially that means calling s8000 m3 tells the spindle you want 0 rpm. You’ve mapped s8000 to the min speed this throws off the scaling. I find it works best to leave that number at 0 or 1