If you set any hotkey slot to "R" and you are flying and sneaking (to fly downward), you cannot hotkey to the "R" slot. I tested around a little and haven't had the issue at all if I set it to other keys (i.e. "G", "E"), and it seems to affect any hotbar slot. When you click "R" then, just nothing happens.
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I have just figured out that this is a hardware limitation.