Playing on a 10-player Realm, after approx 30 minutes gameplay, leads have stopped attaching to a particular Lama.  When right-clicking with a lead in the main hand, instead of the lead attaching to the Lama, the player rides the Lama instead.  No possible action seems to be able to make the lead attach.  The problem persists after closing and restarting Minecraft, and even after re-booting Windows.  However, this appears to be a problem with only one particular Lama, the same lead will attach to a wandering trader Lama.