The problem with PIPBUG has been found, and the problem was nothing to do with PIPBUG!
The first 18up5 board that I built has always worked without issues, but the second “pretty” board did not. It took an embarrassingly long time to notice that the second board was missing the interrupt jumpers JP2 and JP3, and that these were also missing from the instructions.
Once these were installed, everything workied again!
If you are having problems with the G command not working, then make sure you have these jumpers installed. These prevent the CPU from getting an immediate (and unhandled) interrupt whenever a user program is executed.