Hotbar is full of random items. (In particular I was playing with the purpur blocks and beacons).

Placing them down works fine. I then go into Inventory and hover mouse over a block in slot (2-9) and type "1" on my keyboard to switch them.

Placing the block does not work. It briefly looks like the new item but then swtiches to what I originally had.

Using just the mouse to drag the hotbar items around works fine. It's using the keyboard option that doesn't.