Okay, so I saw this theory once and it totally makes sense to me. So in Avengers Infinity War when Thanos arrives in Wakanda, Captain America calls all available heroes to fight.

After holding back the gauntlet, Thanos delivers a blow to Cap's head. It is seen later that Cap is perfectly fine. In the moment, I passed it off as just waking up from being knocked unconscious.

But then as I got to thinking, I realized that a punch like that would've at least left him out of it for longer then five minutes, right?

Well the theory is that the punch was a fatal blow. The punch delivered by Thanos did indeed kill Steve. But when Thanos turned back time to revive Vision and get the mind stone, he unwillingly revived Steve as well, explaining how he's perfectly fine.

Theory is not mine, that is all. Goodbye *bows*.