Discovered this on the latest version of the ATM to the Sky 10. When using the Anvil on a stick, it lists the correct Raw XP on the upper left of the interface. However when you actually combine the item in the anvil, it uses full levels instead of the raw XP amount.

For example, to add Fortune V+Efficiency V to a netherite mesh costs 15 levels. If I have 30 XP levels, and I use a normal anvil to combine the mesh with the book, I drop to 26 levels. However, if I do the same thing with an Anvil on a stick, I am dropped all the way to 15 XP levels.

I discovered this by accident when I had well over 400 levels collected meticulously from the quest book, and did a ~40 level anvil recipe on the Anvil on a Stick thinking it would be super convenient, only to find to my dismay that I had wasted hundreds of thousands of raw XP!

Not really anything I can do about that. I'm not going to mixin to every mod which adds their own flavor of anvil to apply the alternative XP mechanics. It's simply not feasible from my end.