When listening for SpawnChangeEvent in a plugin, the listener method is not called when the command /setworldspawn is executed, and I couldn't find any version of Spigot where this worked.
After a bit of research, a similar issue was opened on PaperMC/Paper repository (https://github.com/PaperMC/Paper/issues/1913).
It appears it is due to the fact that the Event was created before the vanilla command was created.
There's also a PR on Paper's repository fixing this issue (https://github.com/PaperMC/Paper/pull/4177/files), I'm guessing this wouldn't be very different for Spigot.

I've added a .zip containing the plugin I used for testing.

Let me know if you need any more information

Thank you in advance