I also suspect static. It is plastic. It is winter. And Scott tried multiple bits (old/new) with same result.
I haven’t cut HDPE, but I know from experience that when you’re troubleshooting a frustrating problem, you’ll take any ideas/thoughts you can get. Plus, I’ve been dealing with chips extractions issues on plywood. Related issues.
I run a DeWalt router on XXL and get tons of static from it. Especially with extraction hose connected. (Still researching proper method to run copper wire along it to ground.) But static loves it some plastic. Even more in a dry (low humidity) environment. Maybe the static level is strong enough to prevent chips from flying off as expected. Especially on the second, deeper pass where chips are between the side walls and bottom of the cut. They get stuck, maybe adhere to the mill (is that possible?), you get re-cutting. Then, like someone else mentioned, would those tiny chips fuse together to make the clumps? Good luck!