This does not affect vanilla server because the only C2S packet capable of accepting ItemStack (CreativeInventoryActionC2SPacket) rejects item stacks that cannot be serialized to disk before processing.
Fixed in 25w18a.
Fix version is indeed 25w18a.
Can no longer reproduce in 25w18a. Will check if this is the fix version.
Can confirm in 25w18a.
Fixed in 25w18a.
This bug can be reproduced more easily in multiplayer.
Given the changes made in 1.21.5, this could be a valid parity issue.
Appears to be a duplicate of MC-304369