Investigated and confirmed!
I had about 11 gallons of E10 Shell 93 in the tank and filled up the rest of the way (5.9 gallons) with Wawa E85 ($2.59/gal; the picture below is before I filled up).
After filling and monitoring E% in MHD FlexFuel the E% jumped initially from 10% to 42% and then settled down over 10 minutes to 34%. Apparently because ethanol is denser than gasoline the E85 initially sinks below the gas in the tank before mixing, so it gets picked up first by the fuel pump. Eventually it settled down to 34%, so my mass balance calculations tell me that the Wawa E85 today was actually about 75% ethanol.