I have my ninth hotbar slot set to R (for my water bucket) and anytime I type in the creative menu an R, it does not let me type it, and just selects what I've typed.
Edit: This seems to only happen on multiplayer servers and still happens on a 20w21a server too.
Cannot Reproduce. Please make sure that your mouse cursor does not hover over any inventory slot, as it then will take precedent to put that item into the corresponding slot.