So, I have only played through the entire game once and it was a Pacifist run. I did the full extended run, reloading my last save and everything.
I started a Neutral run later, but I haven’t even completed it yet.
Edit: I just googled it. You have to view the neutral ending once, which you get at the end of the Pacifist run before reloading. You do not need to do a full Neutral run.