I found a problem with the beta version of Arduino AVR Boards required by MiniCore for the PB chips:
Arduino as ISP programmer can't be used with avrdude 6.3.0-arduino12 · Issue #11 · arduino/avrdude-build-script · GitHub
Try making this change to the arduinoISP sketch...
case 'E': // extended parameters - ignore for now
{
uint8_t l = getch();
if (l >= 4 && l < 20) { // do a bit of sanity checking
fill(l);
}
empty_reply();
break;
}
case 'P'