You likely have 2 grounds on the edge of the jamma connector, and using 6 button option has this chinese standard.
Use a wireless keyboard to re-do the button mapping. The OS thinks you have a button pressed constantly. (coin or start. i think.)
Normally 'select' key doubles as shift, and shit+some key opens up the game menu.
(On my setup, start+p1b1 does this. I need to double check, i am in hospital right now Back in 48h)
@West I have this running rgbpi-extra running too with CV1K on a standard pi 4 and the slowdown for Futari15 and Espgal2 seems almost spot on. Latest FBNEO core was added this week after i requested it
Edit: Buttons.. I'm not sure what version im on but i think it asked me to enter my buttons on first boot, and didn't auto-set them. Not sure..