The Bug:
When trying to append a NBT tag to the Items list of an Inventory, the NBT tag doesn't get appended and instead replaces the first element of the list
Command I used:
/data modify block ~ ~-0.5 ~ Items append value {id:"minecraft:apple",Count:13b}
How to reproduce:
-Stand ontop of inventory(chest,dropper,dispenser)
-Execute Command multiple times
You need
Slot