In defense of the MSG thing - afaik you can't actually be allergic to it (outside of some super rare conditions, like the ones where youre allergic to things like icewater) but it is a relatively common migraine trigger. Downing it with booze, which for many people helps with migraines (the vasodilator/vasoconstrictor effects sorta cancel eachother out) means it has a minimal effect.

This isn't everyone, obviously migraines are uncommon and ones triggered by MSG are an extension in rarity, but it is explicable behavior in a very real subset of the population.