The bug

When using /loot mine on a chest or other container, only the container itself will be dropped. The custom name, if any, will be preserved—it is clearly that specific chest—but the items inside of it will be ignored.

Similarly, when using /loot kill on an entity, only the entity's death loot table is considered. Any armor items that the entity is wearing are ignored, any saddles, chests, or chest contents are ignored, and non-living entities like paintings, item frames, and minecarts will fail to drop anything at all.

Reproduction steps

/loot mine

1. /setblock ~ ~ ~ chest{CustomName:"\"Test\"",Items:[{id:stone,Count:1b,Slot:0b}]}
   

2. /loot give @s mine ~ ~ ~
   

   The dropped chest has the custom name
   The chest content (stone item) is not given to you

/loot kill

1. /summon husk ~ ~ ~ {HandItems:[{id:stone,Count:1b}],HandDropChances:[1f,1f],ArmorItems:[{id:diamond_boots,Count:1b}],ArmorDropChances:[1f,1f,1f,1f]}
   

2. /loot give @s kill @e[type=husk,limit=1,sort=nearest]
   

   You only receive the normal husk drops (e.g. rotten flesh) but not the hand or armor items

/loot kill (hardcoded drops)

1. Place an item frame and place any item inside it

2. /loot give @s kill @e[type=item_frame,limit=1,sort=nearest]
   

   You receive nothing