I buy my E85 fuel from a local gas station that sells all kinds of fuel from straight standard pump gas to diesel and VP brand race gas of several different octane ratings. The first ten gallons of E 85 I bought was in late fall and it ended up testing to be E75, not E 85
That was the E 85 fuel I used to do the first dyno testing on my e85 Dominator race carb. that wanted to be jetted down leaner to get the best HP and torque. The dyno shop ended up buying ten gallons of E85 from the same station the next week after my first dyno test and left it in the two 5 gallons plastic fuel jugs in the dyno room that winter. I went back the next spring to do some more dyno tuning and they insisted that I use their fuel as they weren't going to use it, I tested it there before using it and it tested out to be E90
I assumed that some of the gasoline had evaporated out of the jugs while sitting in the dyno room
I have seen the same thing now in my fuel sitting in my plastic fuel jugs during the winter in my enclosed race trailer that sits outside all winter
Two winters ago it got down to -25F at night for several days that week, it freezes at night most of the winter here from mid October to at least mid March
My message is it ain't hot in the trailer during the winter so it is not the heat causing the fuel change, maybe it is the plastic jugs
Mine has tested to be right at E85 out of the pump to start with but ends up testing to be from E87 to E90 when it sits more than a month or longer in the plastic jugs