I agree this looks like a tubing defect.

I'm surprised OP can't find a builder for a Keirin-type frame. They're a bit simpler to make than a road frame and much simpler than a rando frame with racks, a dozen braze-ons, internal routing, etc... That said getting the clearances tight and leveling the track ends can be tricky. I once made a frame with a dimpled ST and spent HOURS figuring out the the correct sprockets, chain/chainstay lengths combo to achieve a pleasing clearance.

BTW Kalavinka sells beautiful lugs and ends that fit the Keirin aesthetic.