I dont do many full tang handles, so take this for what it's worth.
My first response would be to seal the inside of the scales after you flatten them, let it dry, then lighty reflatten to knock down any bubbles or lumps, and then glue the scales on the blade tang.
I believe there may be moisture creeping in from behind, manifesting itself around the porous edges. This will allow the wood to swell a little, but it may be enough to pop the finish off. Lin