I can't speak for all EVs but my Ford with a 400v hybrid system (DC-to-DC, no alternator) was able to keep operating perfectly with no 12v battery whatsoever. There was an assembly defect where the positive terminal connecting the battery to the fuse box eventually came partially loose and would disconnect as the engine bay warmed up. It would start up fine and drive with zero issues but then completely black out as soon as the car was turned off.