A friend of mine took in a 10 year old royal a few months ago who has never switched. She has always eaten mice and won't take a rat for anything. She is perfectly healthy and seems quite happy.
I have a male royal who goes off feed for no reason at all. He'll lose a few grams but is still healthy and about as active as a royal gets lol. I think he does it as a self diet as he'll plump up, almost to the point I think about cutting his food back and he does it himself. And he still sheds for me about once every 6 weeks or so.
Royals can just be finicky creatures. I honestly wouldn't worry about him.