Nautilus' ult resets when a champion becomes invulnerable/untargetable.

I recently noticed when playing against an Ekko, I'd ult him, he'd ult, my ult would stop following him and immediately go on cooldown. I also had this problem happen twice against champions using Zhonya's Hourglass. The ult stops following them and goes on cooldown. But the problem is the inconsistency here. I tried using my ult against a Vladimir, he'd use his blood pool, my ult would stop following him, but it would be back up immediately. Not sure what the problem is here, but I'd really like a fix.