Diamonds (and only diamonds) will be moved into the 2x2 crafting slot on any chat message (including the gamemode updated notification, but more generally on regular chat), if you have at least 9 of them in your inventory (regardless of stacking). In creative, they are still moved to these slots, even though the slots are invisible.
They will return to the normal position after opening and closing your inventory.
Confirmed.
For me, it only moves 4 diamonds. Then the bug can't be replicated again until I "undo" it by opening and closing my inventory.