I have the 08 same model.
Mine is and always has been clear
I will fathom a guess. If you get lots of pollen in your area, especially tree pollen when it's heated it gets yellow and film/crystal like.
Would explain the repeatability. Just added new aux lights will let you know if it changes.
Edit ... Should say I think you have pollen intrusion. You ride all day, hot air in bulb, when it cools it sucks in pollen filled air coats everything. Next ride it bakes on and repeats process.
Is you bike outside when stored? Mine is always in garage when it cools and garage has filter system running.