Thing is, the IP is already quite sprawled out, between the movies and the cartoon series. In this case, I think it pays to be consistent. I don't see why a 3rd movie with most of the original cast shouldn't be a numbered sequel. A subtitle would just add confusion at this point.
The MCU I've found to be confusingly inconsistent when it comes to these things. It uses both numbered sequels and subtitles interchangeably, it seems. For example, the first two Captain America movies are numbered sequels, but the 3rd one uses a subtitle, and he's hardly in his own movie as it concerns more of the overall Avengers in terms of plot, rather than something distinctively Captain America. So is it a Captain America movie or is it not? Feels more like an Avengers movie.
I'm not sure what character you're thinking of but there is no Captain America 1 and Captain America 2. It's CA The First Avenger, CA Winter Soldier and CA Civil War. Different MCU franchises use different approaches but they're all consistent within themselves so far.
Iron Man, IM2, IM3.
The Incredible Hulk.
Thor, The Dark World, Ragnarok, Love and Thunder
Guardians of the Galaxy, GotG Vol. II, GotG Vol. III
Ant-man, Ant-man and the Wasp, ?
Dr. Strange, Dr Strange in the Multiverse of Madness, ?
Black Panther, Black Panther 2, ?
Spider-man Homecoming, Spider-man Far From Home.
Captain Marvel, ?
Avengers, Age of Ultron, Infinity War, Endgame, ?