Are watermelons in Thailand injected with a chemical substance?

Still, here is one of the many articles that point out the danger of MSG.
Make your own judgement.
As early as 1968, a study by Washington University showed that excessive consumption of MSG led to damage to brain cells in laboratory animals. In response, MSG was removed from many baby foods. The use of MSG is a risk, especially for brains that are still developing (Lima, 2013). Neurosurgeon and nutritionist Dr. Russell Blaylock has written a book, 'Excitotoxins: The Taste that Kills', in which he explains that the free glutamic acid from MSG, like aspartame, is an excitotoxin. An excitotoxin is a substance that overstimulates brain cells, which can lead to cell damage and eventual death, causing permanent damage (Blaylock, 1994).

Our brains have many receptors for glutamic acid, and in some areas, such as the hypothalamus, the separation between the bloodstream and the brain is permeable, allowing free glutamic acid to enter the brain. This especially happens when there are unnaturally high amounts of free glutamic acid in our blood, such as after eating MSG. The blood/brain separation is not designed for that. If the glutamic acid there reacts with neurons, it can lead to cell death and permanent damage. (Xiong, 2009).[19] This plays a role in all kinds of brain disorders such as strokes, trauma and epilepsy, as well as degenerative diseases such as Parkinson's, dementia and Alzheimer's (Mark 2001),(Doble 1999).

Autism also has to do with abnormalities in the glutamate transport systems. In people with autism, there is too much glutamate in the nervous system. Glutamate blockers are therefore used as medication. There is a good chance that excessive MSG through food will influence the development of autism.