Sounds like a bad seal, you need a new guage. When I bought mine, the ring around the gauge was out of round so I exchanged it ... this is what I was afraid would happen if I didn't.
I have been to a couple different dealers and they say it's normal. Hard to believe you spend all this $$ on a bike and that's the answer. My 82 Suzuki didn't fog up and that was 25 years ago and HD can't get it right in today's world of bikes. What a shame.[:'(]
My oil temp and fuel gage fogged up last weekend. I attributed it to the rains I was riding in, but that doesn't make it right. Could also be a heat differential between inside and outside the fairing, but again, it shouldn't do it.