is it still wet? Hit with a heat gun, looks like it was cold and bubbles were trapped during the drying…
It is probably moisture in the wood.
I’ve had something similar happen to me before on Cherry.
I stained the cherry with Varathane Kona oil stain, and then overlaid a coat of Minwax Natural 209 over the entire piece. It dried fine, but after a few weeks these yellowish or off-white spots showed up. I was able to wipe them off with acetone on a cotton cloth using a light-touch without damaging the stain.
I don’t know why it happens. I thought it might have been the light-colored pigment in the Minwax stain flowing together, but it could also be a chemical reaction between the two stains or between the stains and the wood, or something else entirely. No idea.
i would say there is either something on the wood or in the wood in that spot. the poly is not adhereing to the wood it is just flaking off.
Can also be resin in the wood. Particularly in the sapwood. You can try sealing it with shellac and then putting poly over the top. This usually works well.