I don't know if this is a bug, but it seems like an exploit. Bastions are supposed to be challenging. Players using hoppers to take items from the chests take that away. With the piglin brutes coming sometime, this doesn't seem like it'll be an issue for too long, but it's still there. An easy fix would be to make piglins get mad at the player if they open a hopper. Or they could get mad at a player opening a hopper connected to a chest.