All other containers(ie. chests, droppers, etc) use: {LootTable:"minecraft:chests/end_city_treasure"} in its data if you want to add a loot table to an container. Suspicious Sand on the other hand uses: {loot_table:"minecraft:archaeology/desert_well"}

This code is inconsistent with other containers and I had to go find naturally generated Suspicious Sand to figure out why my coding test wasn't working.

 
User Dhranois in the comments added: 

Also affects the item option: suspicious sand: "item", containers "Item(s)", item entities: "Item"