Specifically the Deltarune newsletter announcing the release date said

In Chapter 4, we saw dark clouds on the horizon…

That’s why, let’s not look there for now. Let’s turn around and watch the sun, before it goes down completely.

Let’s smile again.

Let’s have one more fun adventure, okay?

So the nostalgia theming and all the callbacks to both Deltarune chapter one and Undertale are very much intentional and this is signalling that everything from here on out is probably going to be a bad time.