If you let hayfever fallow through till you sneeze, then when equipping gas mask, it will get stuck in mid-state and the UI will flicker and you won't be able to tell how long until you need to put on the gas mask back on. The UI won't go away when you do this, it will just keep you from completely sneezing if you keep it on.
Eating nettle seems to fix the bug, reloading game won't.
UPDATE: eating a nettle DOESN'T fix the issue, but it lessens it; the hay fever still flickers, but it is gradual like normal. It takes longer to get rid of it using a gas mask so I need to leave it on for longer.
* be in lush season, let hay fever go through * put on gas mask afterwards and take it off soon after * it should glitch, if not read note NOTE: character sneezed once outside, then sneezed inside house, then I put on the gas mask. May have happened when character almost sneezed again.