Huh, he's right. We have a winner.
I can do this in mame. Booting up to test mode yields a pokey error. Resetting while in test mode, or going from game mode to test mode does not yield a pokey error (in mame). And wouldn't you know it? My machine is behaving the same way. No pokey error if I go from game mode to test mode.
Have I been chasing my tail this whole time?
I guess it really is just the Leta.
It's got to do with the difference in execution paths through the code altering the "startup" sequence when it encounters the self test switch being on vs off (to game mode).
I'll send a bill later in the week.

