Never had these issues before unleaded or mixed fuels in the late 70's . Been putting returns on since and solved many carb rich problems. I am not a chemist so I always call it ethanol type fuel. We use to call it corn gas back in the day. But what I can tell you is my Duster had the same issue with 114 and 116 race fuel. That is why I spent the $500 for what I needed to run a return. Big difference when going round after round with no cool can or on the street.
I would always have to start the car and turn the pump off and on until cool fuel got to the carb until it would idle, before the return was installed. Take it to the drive in up the street and it would not restart without doing that .
After the return was installed it never did that again and the car ran leaner that I had to put smaller low speed air bleeds in the carb. Yes we had rich lean gauges in each bank.