How the bug works

While using loot command to replace items on a player's main-hand, it works, but it also clears the offhand via a minecraft:contents entry.

How to do

1. Use the command to place a contained shulker box.

/setblock ~ ~3 ~ shulker_box{Items:[{Slot:0b,id:"minecraft:stone",Count:1b}]}

1. Take an item, (like, an emerald or else,) on your offhand.
2. Use the other command to trigger the condition (see shulker_box.json.)

/loot replace entity @s weapon.mainhand mine ~ ~3 ~ stone{getContents:true}