The enchanting infuser doesn't take into account any modded bookshelves placed around it for calculating maximum possible enchantment level, only vanilla bookshelves are taken into account.

Confirmed with bookshelves from the following mods:

Better End: https://www.curseforge.com/minecraft/mc-mods/betterend
Charm: https://www.curseforge.com/minecraft/mc-mods/charm

Fixed by #9 .