The rear brakes (2013 forumula R1) on my full susser partially locked on today while riding in full sunlight, there was no lever travel at all, and it was noticeably worse in sunlight than when I was under trees. When I got back I lubed the pistons and changed the pads for some more worn ones, it made no difference, I had to let a little fluid out before they worked. Then I went to move the hardtail (which I rode on Sunday with no issues) and that had exactly the same on the rear K18’s despite just being sat still! So I let a little fluid out of them as well. It was only 26 degrees in the garage, it seems bizarre that a relatively small difference in ambient temperature could make so much difference.
If you are riding in the heat, make sure you have some travel on the brake levers before you head out! Those two were always a bit lacking in travel but it was never an issue before.