I am using the Better Minecraft modpack.

I put in an enchanted sword in an anvil in the first slot, then a rune. Nothing results. Wouldn't let me combine. Same with ancient tomes. It only works if I swap them and the ancient tome first then the book, which results in an ancient tome with 2 duplicate enchantments..

I doubt any other thing to do with the anvil would work.

(recording of the issue)
https://medal.tv/clips/57544933/d1337Z4Qc1vK

Some other mod seems to be messing with the anvil for some reason. I'll try and make it so the rune/tome application is set to the highest priority to try and counter it.