I ported the CHESSmate code to a Raspberry Pi 4 running under a Python based 6502 emulator. One thing that I learned doing this is that this emulator runs way to slow to be a viable option. Perhaps a C based 6502 emulator running on the Pi would fare better.

To my surprise, CHESSmate exhibited exactly the same errant behavior as it did running on the ESP32 and the Arduino Pro Mini.  So there is  no way that these glitches are a result of a misbehaving emulator. 

So I guess I'm back to bad ROM images. I guess I'll have to dump the original CHESSmate ROM to know for sure.