Normally, the /loot insert command fails to insert anything into a chest if that chest is full.

However, if the chest's contents already include a stack of any stackable item(s) the loot table tries to drop, the command will say that those items were inserted, even though the contents of the chest didn't change.

To replicate:

Place a chest and completely fill it with an item that isn't bones or arrows.
Spawn a skeleton, and use

/loot insert (X Y Z) kill @e[type=minecraft:skeleton,limit=1,sort=nearest]

to try inserting its loot into the chest. The command will say it drops 0 items every time.
Replace one slot in the chest with a stack of bones, and try the command again. The command will say it's placing bones into the chest (when the loot table happens to roll them, anyway), but never arrows.
If you replace the stack of bones with a stack of arrows, the command will say it's placing arrows into the chest, but never bones. If you add a stack of bones in addition to the stack of arrows, the command will say it's placing more of both into the chest.