I have never had a problem with a Hat figure but have had some very old Airfix figures go brittle and break. I also have figures by Esci, Italeri & Revell and have not had a problem with any of those.
To be fair it was only a few figures (most were early Airfix Highlanders) out of the several thousand Airfix figures which I have, many dating back to the late 1960s and early 1970s.
I do store them in a cupboard in filing boxes so most are only exposed to the sun for short periods (although I do have some in glass fronted display cabinets in my study which seem OK). I have also always used Humbrol paints which seems to help protect them.
I have many Hat figures, but not the particular set which had this problem.