Description:

When an non-stackable item has its "count" value changed from its NBT, it does not display that in a container, such like a chest, but they are stacked, they just don't show the value, unless you take the item with the cursor, which is a strange and inconsistent behavior.

However, it does preserve the number when you take it out with numbers on your keyboard, but causes the item to disappear: https://bugs.mojang.com/browse/MC-278488

Re-production steps:

1. Give yourself a Netherite Sword
2. Place down a Chest
3. Put the Netherite Sword anywhere in the chest.
4. Stand on top of the chest
5. Run: /data modify block ~ ~ ~ Items[0].count set value 5
6. Take the Netherite Sword out by clicking on it with the mouse once which makes the item follow your cursor to move it to a different slot.
7. Observe how it says "5" on the Item, but how it didn't have a number before at all

You can additionally test it with the Test Game that I attached.

Bug Repro NBT Count Chest.zip

Expected Result:

I expected that the Swords would show a 5 next to it.

Actual Result:

The sword only showed a 5 once I took it.

nbt_count_bug.mp4