The Bug
Suspicious Sand contains a specific Loot Table to decide what can Players get. However, If a Sus Sand contains any Loot Tables related with Bastion Remnant, you'll get "War Pig" Advancement.
How To Produce
/setblock ~ ~1 ~ minecraft:suspicious_sand{loot_table:"chests/bastion_treasure"}
Use a Brush to brush this Sus Sand - No need to brush completely
(×) "Advancement Get! - War Pig"
The Third Party Source
The conditions for the advancement are just to trigger any of the bastion loot tables. There's no check for actually being in a bastion when doing so.
Seeing as this can only be achieved with commands, this is likely not worth fixing.