As per the information provided on https://www.minecraft.net/de-de/article/minecraft-snapshot-24w10a, the slot name player.cursor is not intended to function in creative mode, particularly within the regular inventory. However, this restriction doesn't extend to GUIs other than the regular inventory. When examining a GUI other than the standard inventory, the slot name does indeed work in creative mode. To replicate this, input the command
item replace entity @a player.cursor with airinto a repeating command block. You'll observe that items picked up with your cursor in creative mode are deleted when viewing a chest, crafting table, etc.
Thank you for your report!
After consideration, the issue is being closed as Working as Intended.
Please note, that mechanics of the game may change between updates.
Things such as graphics, sounds, world creation, biomes, redstone, villagers, and animals may not work the same in current versions.
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