I have been updating a server that uses a custom dimension in a datapack.
It worked throughout the Vanilla snapshots and Release Candidate 2.
It does not work in Spigot.
Spigot generates a folder path in the "/minecraft/world_kingdoms_spawn" directory that custom dimension datapacks ignore.
Usually the directory for the dimension would happen in the "/minecraft/world/dimensions" folder but instead tries to put itself at "/minecraft/" folder.
I cannot guarantee that it is not my fault, but I tried it several ways.
I am solid enough to upgrade 4 out of 5 servers... but this one doesn't seem to work with spigot.
It could be just something simple. but I will bash a head at a brick wall all of tomorrow!
I appreciate the Spigot team, take a bit of a break!!! Good work!