What reasons could cause my Bosch dishwasher to periodically fail to fully clean dishes?
After a few months of rerunning failed washes, I found the issue... I hope.

I disassembled the dishwasher's front and removed the control board. Upon close inspection there were two dead/fried ants (yes, the insect) on the bottom of the control board near the pins for the relay the heater. This relay is already unreliable in this dishwasher model and I have fixed it before.

One of the solder joints connecting the relay looked like a cold solder joint. Cold solder joints can sometimes have intermittent connection issues so I was hopeful that this was the issue. I suspect the ants were in there during a run and caused the pins to arc which melted the solder joints.

I removed the solder from the joints and re-soldered the pins. I reassembled the dishwasher and have had two successful runs so far. Fingers crossed this resolves the issue.