i still think your settings are just running too hot.
fwiw, i run standard maple ply settings with BB and single layers of masking and i have no issues with flashback on the back side. i’ve got some custom settings i’ve used occasionally when the sheets were slightly thicker than .125" instead of slightly thinner. and it’s always important to note that .125" is nominal thickness and i’ve had BB anywhere from .117 to .132. and that difference is enough that just using the maple ply setting doesn’t always cut through with the thicker ones.
is it possible you’re not getting a good adhesion with your masking?
sometimes if the sheet is either a little dusty or the grain is really pulled up (so quality dependent on the surface), you can wipe down the sheet first or even give a super quick buff with a random orbit sander before putting the masking on and it holds better.
the other trick is to make sure you’re using something to “burnish” the masking onto the wood. even something as simple as the edge of an old credit card is all you really need. if it starts to pull away even a little, you can get flashback on the back or burn under the masking on the front.