EV20 is a different story though.
EV20A takes place before EV21, while EV20B takes place after/in the middle of EV21, so it makes sense it seperated into two crystals.
EV04 and 05 is right next to each other, so combining them into one is logical.
Aside from moving Arika between "Maps", EV41/42 is pretty much like EV04 and 05.
What I'm saying is that's the way TH set it up, even if it doesn't always make sense. EV04/05 take place on the same Map.json (it's the entire 2nd story of the hotel), so it doesn't need to load in anything else. EV20A is on a different Map.json than B. EV04/05 is
two events scripted to be one, whereas EV20A/B is
one event scripted as two
In fact, I think EV41/42 was originally supposed to be the same event, look at the Arika pov for EV41, the last line is Kurt saying 「.......はっ!」. This is the also the first line he says in 42, but there now are some extra lines in front of it because TH probably set up some kind of transition in between 41/42, which makes it so they're on different crystals.
And not for nothing, but if they were one event, the whole thing would be an hour and half long, which would be way longer than any of the others