To be more specific - while on Sienna, if you bring yourself to high overheat, you get a slowdown debuff on your character. These debuffs are supposed to apply when you’re over a certain heat threshold, and disappear once you go below that threshold, whether by natural venting or manually venting. This bug causes the player to retain the slowdown from being on high heat even when they manually vent themselves below that threshold, i.e. low orange heat. Once your heat goes gray the slowdown disappears. Tested and confirmed the bug on pyro/bw with every staff.
The bug is most noticeable if you bring yourself to max heat with your staff and vent down to low orange.
Here’s a video showing the bug, and the steps to reproduce it: