It looks like the flutes were just getting filled with dust and the cutting edge was no longer biting into the wood. This caused the tool to push the wood instead of cutting through it.
I switched to a tool with a deeper spiral profile, 1/4" diameter and 2 flutes instead of three. The chips were bigger and the heat dissipation was much better. In the end a piece got stuck to the bit as it was lifting out causing the bit to break in the rapid to the next piece.
Live and learn.