And, that was it!
- Bingo. Replaced the chip and that was it. The little blob in the top left of the LCD is the 1802 writing FFs over spi.This was just tragically difficult to debug because it depended on what had been written to the latch right before I used it so it was corrupted by all sorts of extraneous phenomenon. I kept seeing these little glitches and trying to fix floating grounds or bypass capacitors, swapping chips blah blah blah. As I say, I think this is what stopped me on the breadboard so getting the PCB made just reduced the number of variables to something manageable.I'm glad that's over - on to the next problem.