worm castings aren't pure N, that's not the issue. When you put a cut into compost, it tends to compost. A cutting is not far from being compost already, because it isn't sealed. Roots aren't like drinking straws. To get in, something has to cross a barrier. Bacteria stay outside for the most part. When something alive gets inside a root, we call that "INfection", even if it is something good like arbuscular mycorrhizae.
A cutting, on the other hand, is just like a drinking straw - open at both ends. Anything in the soil solution is gonna betaken up via capillary action.
See the difference? Your plant's roots LOVE worm castings and all the cast of characters living in them, but an open stem? not so much.
OFC Approved!
This explains why my lavender cuttings are all dead, bar 2. I made my own blend with my own compost and a soiless medium...
I will use perlite next time for sure. We cant get the napa stuff in Oz.