Fires set with magic (i.e. burn action) will not function properly when set. Even if there is no undo/the spell is committed, fires set with this spell do not age, extinguish, or spread.

What version of Spigot are you on?
I'm wondering if this is some new behavior, maybe fire blocks have some new properties....

Or maybe I'm mis-remembering, but I thought fire would spread naturally, even with undo.

I'll try it out with the builtin fire spell when I get a chance, maybe tweak it to last longer.

I certainly remember doing a tremendous amount of work to properly tracking fire spreading so it can undo, anyway 😂

I'm currently using Paper for 1.20.1, Build 196, but I've noticed this behaviour back in 1.19 and 1.18, and possibly 1.16 as well. Fires set with magic don't seem to spread at all (as far as I can remember) whether undo is set or not, and if they do not undo they remain forever as though they were on netherrack. It could potentially be something changed back in 1.16 when Soul Fire was added? I recall trying to use Soul Fire in spells and it has always had some strange functions (issue #1131) so I'm unsure if normal fire's functionality was changed at that point as well. Or it could potentially be an issue with the "infiniburn" types of tags for blocks that came along with the new worldgen, but that's a shot in the dark. 😅