I enchanted an iron sword with Smite II. I had a book for Sharpness I, and the option for obtaining a Smite II Sharpness I iron sword did not appear in the anvil's output box. I tried this in creative mode as well. I also tried combining a Sharpness I iron sword with a Smite II iron sword. Nothing appears in the output box; i.e. no "you don't have enough levels" indications or iron sword icon.
I also did a little experimenting with more books to figure out where the game's internal logic might be going wrong; I enchanted a Blast Protection book with a Sharpness book and then tried to apply those enchantments to an already enchanted iron sword. Interestingly, Blast Protection carried over but Sharpness did not.
I do not know if this bug applies to any other enchantments, I only stumbled upon the bug regarding Sharpness and called it quits on playing today.

P.S. why is it called buffet mode if you can only select one biome? kinda bull