>>714330384
Gaster absolutely intervenes for things we aren't yet meant to know at that point in time, like how Onion completely disappears in Chapter 4 if you've been meeting up with him every applicable chance from the very start, and returning to the spot after trying to wait nets you the appearance of Gaster's strongest soldier.
Adding onto that, when you return to that spot post-Dark World and sit down with Susie, the song from across the sea starts playing, as evidenced by Susie commenting on it, but (You) aren't allowed to hear it.