Found it (or something credible on the same subject). And I had it backward -- the cause is less ethanol fuel refining, and the effect is also hitting beer production.
"Most of the ethanol produced in the US is blended into gasoline and used as a fuel. As a result, CO2 supplies is linked to fuel demand and production."
Wait, wut? Mushroom shortages too?
https://www.fooddive.com/news/4-ingredients-facing-shortages-amid-the-pandemic/579323/