In " * " is the thread before the update
*I joined save of my test world ( outdated ) where and make system
which give me the items and then remove them.
When i on the creative item has been removed but when i on survival or adventure this stays on inventory i noticed that this item is "ghost item" :
1. when i touch them it's remove.
2. I can't drop them (Q)
As if command block remove them too fast and minecraft did not manage to update the inventory.*

Steps to Reproduce:
I attach the screen of the command block mechanism
1. Add scoreboard:

/scoreboard objectives add inv_lock dummy

2. Replace Head slot with helmet
In Repeating command block paste this:

/replaceitem entity @a armor.head minecraft:leather_helmet 1

3. Test for helmet count
In first Chain command block paste this:

/execute store result score @a inv_lock run clear @a minecraft:leather_helmet 0

4. If helmet count is 2 or more delete them
In second Chain command block paste this:

/execute if entity @a[scores={inv_lock=2..}] run clear @a[scores={inv_lock=2..}] minecraft:leather_helmet

5. Testing
If you on Creative:
When you drag helmet and drop them to other slot this has been deleting
If you on Survival/Adventure:
When you drag helmet and drop them to other slot this stays on slot where you drop them but when you touch them this has been dissapears.
If you drop them to hotbar and select them you can't drop them (Q)
Something like "Ghost Item"