Typescript async/await cannot determine correct return type
While I havn't found an exact solution, I have found an explanation.
I've looked through the mongoose implementation of the Aggregate class and they've added the then and catch, functions to the Aggregate class to make it act like a promise. This will make the async/await operators treat it like a promise, since they treat any object with a then-function as a promise. This is the reason the Aggregate-class seems to unwrap when returned from an async method.
For the specific problem at hand, as a workaround, I just moved the async part of the method back to the calling code. Basically like it was before I started tinkering with it. It isn't perfect, but it works.