I'm not sure how rare it is, but as an anecdote when I was a teen I knew a kid who had a speech impediment and slurred their words really bad/was kind of mush mouthed.

We were on a group trip for about a week and had forgot their toothpaste and had borrowed someone's 'natural toothpaste' stuff. By the end of the week they were far more intelligible in speaking and were stating it felt like they mouth deflated. They had an undiagnosed allergy to toothpaste their entire life and no one had recognized it.