It's not a good thing or a bad thing; is not (cannot be) a thing at all because it is nonsense. A contradiction. Gibberish.
_
glass.
First things first: Magic.
Second things second: There are more dimensions to reality than we can perceive, and at this time we've gotten to a point of relative certainty that it is absolutely so. The idea that a self-contained reality would be incapable of nesting other realities because it exists within a separate outer reality is nonsensical, at best.
If planes of existence are a material reality and a segment of a plane of reality can be contained within a given structure then any number of alternate planes of existence can also be contained within the same structure so long as they can be contained at all.
Plus, like, C'mon. It's fiction with magic in it. Even if it seems contradictory it still works because that's how it's written and designed to work. suspend that disbelief!