I got this problem and can't seemed to find any articles regarding it on the Internet.
I borrowed a 50mm prime from a friend to try on during my last getaway trip. And found out that there is this circular stain-like marking right in the centre of the lens when it fogged. Found out by accident when I didn't realize the air conditioned room was much much lower than the temperature outside causing the lens to fog noticeably.
I don't know if its a stain or not in the 1st place, but there is this clear circular mark which had a different degree of vapor formed on the lens when I breathed on the lens. However, the lens looked perfectly fine with the naked eye when the vapor evaporated off.
Owner of the lens didnt know what was it. I'd check with a friend and he says the lens is at the initial tags of molding. Seemed like some coating issue to me but it doesn't seemed as bad as those stated on the websites.
Would like to hear if anybody experienced such problems with their lens and if there r any solutions to rectify this.
Pretty pleases and thanks for kind responses