The loot table function minecraft:exploration_map only works when used in the loot table of a container block, like a chest or dispenser. In all other circumstances, including

• Container entities, like chest minecarts
• Fishing
• Death loot tables
• Advancement rewards

the function has no effect, leaving a normal empty map.

Presumably, this is because the function is only able to get the current location when activated from a plain block.