Can't add a favorite avatar as Fallback, but is listed as a fallback

I play on Quest 2, and one day, I noticed when I was changing my avatar that I could set some to a fallback, but when I click on it, it changes my main avatar instead. Is this a bug or is this what’s supposed to happen?

You can only use avatars from public row and your own uploads that have been uploaded as fallback avatars.