Spraying Timbor can, if enough chemical is concentrated at the surface, provide a barrier to the entry of new insects. If the wood develops a crack, there will be untreated wood exposed.
When spraying, you can determine the amount of wood you sprayed and the amount of chemical used per square foot or per cubic foot of wood. In other words, a single, light spraying provides limited protection from a new infection and no benefit for existing activity. Of course, the planing removed the timbor, which is now in the shavings.
But, even if you did get a new infection, it would be weeks before seeing the exit holes of any new insects. So, we know that the new powder, called frass, is from old insects. I assume you are certain that the frass is not just frass from the old holes, but that these are new holes. I assume that the polyurethane was thick enough and covered the old holes you exposed when planing. Can you see the new holes that now come through the polyethylene and the sharp edges of the holes, using 10x magnification? This assures us that they are indeed new holes, as a thick coating of poly would have filled or partially filled the holes. Of course, a very thin coating is likely not 100% effective on new insects.
If indeed you still have insects in wood you heat treated, then you apparently did not reach the magic temperature of 133 F. In fact, in pine, you could have ambrosia powderpost beetles, old house borers, and more.
What is the size of the hole?
Because you planed the wood, did you expose long tunnels that would have been near the surface? Is the frass coming from these?