It's an input polling issue and totally my fault that I didn't catch it before submissions, but the auto-move bug happens almost constantly in Chrome and Edge, and I didn't catch it because I only use Firefox. If you can, use Firefox as the bug only happens in very specific scenarios (defocusing the game window while walking) and can be undone somewhat reliably. I left a note about the issue at the top of the description.
My bad
Edit: Miz gave the greenlight to upload fixes for gamebreaking issues so I've gone ahead and done that.