Some form of glare was my inital thoughts too, but don't have a second lens to test with it just now.
What's really strange is the out of Camera JPG's are not affected and only the raw files as i shoot both raw/jpg at the same time.
Have tried it in a few different software packages like camera raw/ lightroom and on1. They all have the profiles for this lens so its not down to software i don't think 😕