It might be the taste of MSG-rich food, when a sensitive individual hasn't eaten recently, that triggers it. This could explain why it's so commonly reported with miso soup - it's served at the beginning of the meal. The association is known by folks who grew up eating MSG natively: https://twitter.com/kenjilopezalt/status/1291882403506475008...
If this were true, it'd explain why other glutamate-rich foods don't have the same reputation. It might suggest that we shouldn't buy doritos from a vending machine when we need a snack, though.
Studies where respondents can't taste the MSG don't show an effect, so maybe it's the taste, and not the chemical, that causes the problem. Also it seems unlikely that the MSG could actually get to the brain.
I'd love to see an experiment where self-identifying sensitive individuals either eat miso soup or gargle it without swallowing and see if there's a difference in reactions.
I'd also like to know more about common knowledge about MSG in Japan and China.
(I think the racist myth could cause a psychosomatic effect for people who aren't sensitive)
I've wondered this myself over the years. My current operating theory is that it's not necessarily the MSG but perhaps the Mushroom powder...