Darius ult doesn't cast (And other Ults sometimes that are targeted)

The bug occurred after I attempted to ult 2 people who were low in a teamfight in midgame. The damage went through but jinx got the kill both times (yet it reset both times). After it went on cooldown, in multiple fights I would attempt to cast my ult on various enemy team members when the time was right and the animation wouldn't even start (and these people weren't dying either.). I didn't think anything of it, just figured something was weird. Then the last teamfight, my only death the whole game, happened and I got five stacks on naut and moved to go after cait and lulu since they were caught out. I got them to the appropriate health and tried to press r on them. Nothing happened. They both ran away free, I was low at that point and got dove by ekko, and again, I managed to survive long enough to get him low enough to dunk. Pressed r. No dunk. Then death.

I'm aware of there being an occasional bug with targeted ult abilities, but I haven't seen this same bug for at least a year. Would be nice if it could be fixed once and for all. And no, my R key is fine. I even checked in game and turned off smart cast. A range indicator came up.