Empty collection after manual JSON import

I'm new to PL and Mongo. I've imported 2 JSONs like, let's say, Categories and Posts, which are in a relation to each other. Now I can cross-see them in PL as a choice in a relationship field, but both collection lists are showing empty with an XHR response like:

{
    "docs": [],
    ...
}

MongoDB Compass showing all entries, and I've tried to

both collections with no avail. It must be something very simple I'm missing here...

To answer my own question:

must also be imported if versioning is enabled