I had this happen once before on my E-M5 II in Greece with green spots and again yesterday with my VF-4 in Bognor!. If you use high diopter correction as I do and are unlucky enough then it can happen in very bright sunlight as was the case today. I’d recommend that you keep the evf side to your body at all times in strong sunlight especially if you use high diopter correction. I inadvertently reversed it when changing lenses, only for a few seconds at the most and now I have large yellow streaks/blotches in the VF-4 EVF. As the VF-4 is detachable it can at least be replaced, not so nice with a built in EVF as it’s a repair job.
Kind regards, Leigh