Exploration map not working in barter loot table

Since there is currently no item or way to find nether fortesses (other than wandering around and relying on luck), I wanted to add an exploration map to the nearest nether fortress into the loot table of piglin bartering... except it fails to validate whenever I try.

 

The EXACT same block of code works fine in literally every other type of loot table (fishing, gift, chest, entity, even block), but does not work in barter loot tables.