When the Decorated Pot is copied from the world (as a block) to the hand in creative mode, the pot in the hand will contain the contents that were in it the last time the world was loaded.
If the contents are changed, the contents of the NBT data for copying the pot will not be affected until the next world load.

How to reproduce:

1. Place a Decorated Pot and put a few items in it
2. Save the world
3. Load the world
4. Pick the Decorated Pot in creative mode with the pick block button (middle click by default, you will notice that the pot appears in a different slot)
5. Run the following Command to check the content:

   /data get entity @s SelectedItem

Expected Behavior:
The output is:

<Player> has the following entity data: {count: 1, id: "minecraft:decorated_pot"}

Observed Behavior:
The output is (with 4 potatoes and inventory slot 0 as example):

<Player> has the following entity data: {components: {"minecraft:container": [{item: {count: 4, id: "minecraft:potato"}, slot: 0}]}, count: 1, id: "minecraft:decorated_pot"}