It could still be the MSG for some people. My post-doc research was related to Parkinson's disease. Glutamate toxicity is a recognized aspect of the disease [1]. Furthermore, glutamate is a neurotransmitter. Many of the receptors hit by glutamate are targets of anti-migraine meds [2]. Sure, for most the dietary glutamate may have little effect on blood glutamate concentrations. However, if MSG does readily enter the blood of certain people, migraines could certainly be a possibility.

1. [Glutamate-induced excitotoxicity in Parkinson's disease: The role of glial cells](https://www.frontiersin.org/articles/10.3389/fnins.2020.5855...)

2. [Glutamate and Its Receptors as Therapeutic Targets for Migraine](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5935645/)