Hi,
That's probably coming from Iron's Spellbooks itself since I'm testing with Property Modifier because it's not working through kubejs either.
I've generated the list of armors through your mod to be sure to use the correct id but no, it's simply not working.
[items.armor.defense] "irons_spellbooks:wandering_magician_helmet" = 10
When starting back the game, the armor value of this armor piece is not changed, might not be fixable but wanted to still let you know :)