If you middle click a block you don't have in your inventory to select it in creative, and an item in your hotbar is displaced into your inventory due to this, if you switch to survival and try to select it, the item will disappear.

Similarly,

If you middle click a block you DO have in your inventory to try to select it in creative, and an item in your hotbar is displaced into your inventory due to this, if you switch to survival and try to select it, the item will transform into the item you middle clicked.

Steps to replicate the first instance:

1. Fill your hotbar with items.
2. In creative, middle click a block you don't have, so the item goes into your inventory.
3. Without opening your inventory, switch your game mode to survival.
4. Open your inventory and try to grab the displaced item.
5. The item will disappear.

Minecraft 1.20 - Singleplayer 2023-06-08 15-55-43.mp4

Steps to replicate the second instance:

1. Fill your hotbar with items.
2. In creative, middle click a block you have in your inventory, but not on your hotbar, so an item is displaced into your inventory.
3. Without opening your inventory, switch your game mode to survival.
4. Open your inventory and try to grab the displaced item.
5. The item will transform into the block you middle clicked.

Minecraft 1.20 - Singleplayer 2023-06-08 15-56-29.mp4