In the update log (https://www.minecraft.net/en-us/article/minecraft-snapshot-23w06a), it said 

• Stack can be changed with commands by setting slot inventory.0

However, it cannot be changed by setting slot "inventory.0", instead, it can be changed by setting "hotbar.0".

This may be due to forgetting to map inventory.0 to the slot index as what https://minecraft.fandom.com/wiki/Slot shows.

How to reproduce:

• summon an Item Display and set inventory.0:
  • /execute summon item_display run item replace entity @s inventory.0 with dirt
• The item is not dirt.

• summon an Item Display and set hotbar.0:
  • /execute summon item_display run item replace entity @s hotbar.0 with dirt
• The item is dirt.