It's not that different from saying "I believe it will rain tomorrow, and the fact that I believe that is evidence that it is rain tomorrow, so I'll increase my degree of belief. But wait, that makes the evidence even stronger!".

This is completely different. My belief about the rain tomorrow is in no way evidence for actual rain tomorrow, as you point out - it's already factored in. Tomorrow's rain is in no way able to affect my beliefs, whereas a sorcerer can, even without mind tampering. He can, for instance, manufacture evidence so as to mislead me, and if he is sufficiently clever, I'll be misled. But I am also aware that my belief state about sorcerers is not as reliable because of possible tampering.

Here, by me, I mean a person living in Faerie, not "me" as in the original post.