1.14.4, 1.15 Pre-release 6
When binding mouse thumb buttons to hotbar slots, they don't act the same way the other hotbar hotkeys do. You can't put items from your inventory into your hotbar with the mouse thumb keybind.
works like default hotbar buttons inside of the inventory
Doesn't let you move items
How to reproduce:
1. Get a mouse with thumb buttons in the side
2. Bind a mouse thumb button to a hotbar slot
3. Try to instantly put a stack of items into that slot in the creative inventory(To clarify this also affects survival mode)
Scenario: You're in a UHC game and you get knocked off a short skybase. You have barely enough time to open your inventory and hotkey your water bucket into your hotbar. You try to put in your 8th hotbar slot using your handy dandy mouse thumb button. But nothing happens. The water bucket does not go to your 8th hotbar slot like it would if you had hotkeyed it into your 1st hotbar slot using the 1 key above the q. You scramble to put the water bucket into a hotbar slot, but alas, you are too late. You fall on the ground and die, losing the game. End scene.