Despite chiseled bookshelves rotating and mirroring their facing direction correctly, their slots are not mirrored when it would make sense to do so. As a result, chiseled bookshelves break the symmetry of a mirrored structure.

Reproduce the Issue:

Save a chiseled bookshelf with top left slot filled as a structure via a structure block, load the structure with mirroring, notice that it's still the top left slot, and not top right that's filled: