Yes! My son who has autism would eat anything we put in front of him until age 3, when his weight, appetite and health suddenly and alarmingly crashed. Ever since that episode, he's had a much more restrictive diet and food preferences. Night and day.

They never successfully identified what happened. Just diagnosed it generally as failure to thrive.

There’s some research on sudden onset autism being treated with antifungals; so at least sometimes a sudden change may be the result of something very specific in the gut.