Step to Reproduce:
1.Take a slate in the creative mode inventory and place on the floor in the pervious version
2.Exit the game and update to the latest Beta/Preview
3.Rejoin the world and check the slate
Obversed Result:
The slate is disappeared in the creative mode inventory, but it can still be used in the game while in the inventory.
Excepted Result:
The slate is disappeared in the inventory and the slate block turns the air.
Linked issues
is duplicated by 2
Attachments
Comments 4

This issue has been resolved as Won't Fix.
The Slate item was never intended to be available in the game when the Education Edition toggle is off. That it was present in creative inventory was a bug (see MCPE-187696), and it was fixed in 1.21.50.28 Preview/Beta.
"Fixing" this issue would involve either making the item (including its texture) available again, which is contrary to design, or deleting any instances of blocks and/or inventory items of that type, which is something Mojang never does (there's a policy against ever intentionally deleting something a player built). In contrast, making the item unavailable only causes its blocks to become inert and harmless, so it's safe to leave them or leave it to the player to get rid of them.