Why do bubbles keep coming back after I torch my flood coat?

Because they're coming from the wood, not the epoxy.

Air bubbles have only two sources: air you mix into the epoxy, and trapped air released by a porous surface such as wood or concrete. The torch or heat gun is only meant to pop the mixing bubbles, and you have 20 minutes after pouring to do it. If the surface is 100% sealed, no new bubbles appear after that. If bubbles vanish when you heat them and return seconds later, or are still rising 30 minutes to an hour after the pour, the surface wasn't fully sealed.

Certain wood species, rough wood, and old reclaimed barn wood can need 2 or 3 seal coats of UltraClear Bar & Table Top Epoxy before the flood coat. Brush on additional seal coats every 4 to 6 hours until the surface is completely sealed, then wait 4 to 6 hours and pour the flood coat. If more than 10 hours pass between any of these coats, scuff with 220-320 grit and clean with denatured alcohol or acetone first.

Once bubbles have cured into the flood coat, don't try to heat them out. Let the epoxy harden, sand the bubbles out, finish with 220-320 grit, clean off the dust, and pour a new flood coat. If you sand through to wood anywhere, seal those spots again first. With a full layer of epoxy under the new pour, the only bubbles will be the ones you mix in, which pop with the heat gun.