OK, on further analysis...
You also added a CALL CLRSCR to MAIN0. Interestingly, if I comment that out, the old value works. However, if I revert MEMSET to the 1.2.8 version, it also works with the old value. Something with these changes is what caused the problem. I don't understand nearly enough to know why.
Edit: Also, I noticed something. When I'm using jzintv in PAL mode, I very quickly see the Mattel EXEC titlescreen when I reset. I'm not sure if it always did that before, but thought I'd mention it.
Further on this, perhaps it's time we all extirpate the EXEC boot sequence completely. I just took the plunge last week-end with P-Machinery: hooking up at the PlayCable monitor in location $4800, hijacking the initial minimal EXEC sequence. At that point, I do all my normal start-up, but I first start by clearing all memory and resetting all devices (PSG, STIC, BACKTAB, etc.).
Now I see a lot less flickering and color changing when I start up or reset the emulator, including no more occasional appearances of the Mattel title screen. It looks a lot more stable, since I keep interrupts disabled and the display inactive throughout.
I would even recommend intvnut to add this to CART.MAC in a future update to the SDK-1600 (or I can make a patch for it).
It's time to take the training wheels off. Buh-bye EXEC.