I would think its the alcohol/ethanol as well.
Reason being, the custom AI tank I made I ahd lined by my good friends radiator shop. They do a tonne of bike tanks, old car tanks, everything with this stuff, its a type of epoxy but unsure of its exact composition. Its red
I tested a chunk of it before he did the tank for me. Took 3 pieces, left one in 50/50water/meth, one in 100% meth and another in gas...just because. The 50/50 piece didn't degrade quickly, but still turned a milky colour after 3 months. The 100% turned whiteish in a matter of days, the gas one is still unaffected.
I wanted an idea of what I could run for AI with these tests, and those were my findings. The meth/alcohol absolutely breaks down the "common" materials used to line tanks. High percentages of ethanol would do the same thing I'd imagine.