Maybe I'm doing this wrong, but I've tried this on chests in a bastion and also tested it on a buried treasure chest in the overworld

A chest will not empty into a hopper unless you open the chest first.  This defeats the purpose of using the hopper to empty the chest - the point of using the hopper is to not aggravate the piglins by opening the chest.  Maybe this isn't a bug, but based on the excerpt below from https://minecraft.gamepedia.com/Piglin, it seems that the emphasis on opening hoppers around piglins implies that it's a bug: 

"Piglins are provoked when players attack them, break or open a chest, trapped chest, ender chest, barrel, or a shulker box near them. They are also provoked by a player breaking blocks of gold, gold ore, gilded blackstone and nether gold ore. However, they will not get provoked if a player near them opened other containers such as hoppers, furnaces, brewing stands, etc."

I'm assuming the issue is that the loot table isn't generating items unless the chest is opened or broken (breaking a loot chest before opening releases contents) - both of which agro the piglins (unless you use something like TNT, which I haven't tried yet)

Steps to Reproduce:

1. Find a naturally generated chest
2. Place a hopper underneath the chest
3. Open the hopper
4. Notice the hopper is not emptying the chest