I was putting together a few more 8042 boards since the original order netted me 5 PCBs and they were panelised 2 per PCB. One didn't work with the same EPROM as the others. Today I compared it with a working board and noticed that the crystal capacitors were supposed to be like the 3D rendering above, but I had soldered them in rotated 90°. In fact the silkscreen has lines indicating the orientation, and the labels are also a hint. That was dumb of me. A few minutes later it was fixed and worked straight off.

Solved a puzzle, a nice holiday season present for myself! Happy holidays everyone and stay safe!