You are precisely right. It seems to have been protective against a disease found from mouldy rice: Mycotoxins, from toxin-producing fungi found in moldy rice,

You are precisely right. It seems to have been protective against a disease found from mouldy rice: Mycotoxins, from toxin-producing fungi found in moldy rice, can be converted by the host ALDH enzymes from protoxin to toxin, and the effects of mycotoxins can be further potentiated by ethanol. If the incidence of hepatic disease found in many East Asians, especially JPN, is related to the consumption of mycotoxins, then individuals carrying the deficient ALDH22 variant with lowered alcohol consumption would be selectively favored.* https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1821113/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1821113/