MSG-sensitive people can eat mushrooms and tomatoes without ill-effect because the glutamate in unadulterated plants such as tomatoes and mushrooms is bound up in protein with other amino acids. It’s not free. It’s not manufactured. It doesn’t contain the unwanted side effects of production called impurities.
When those same people suffer reactions to MSG, they’re reacting to an unbalanced load of the manufactured (made free) free glutamate that cannot be freed from protein without spawning impurities.