If you have a bbox filter enabled, then overpass can’t find relations which only contain other relations.
For example, this query fails, even though the bbox is set to -90,-180,90,180, which covers the whole planet.
However, if you remove the bbox from that query, then it works. Is there any workaround for this issue?
That isn’t a valid multipolygon relation and I doubt that Overpass has code to determine a geometry for the object in such a case (it would have to recursively determine it from the members).
You probably could restructure your query to select on member geometries, but that wouldn’t make the issue that the object isn’t valid go away.
The Overpass API simply considers the subsumed geometry of all way and node members as its geometry. By definition is a relation without members considered as non-spatial.
The rationale is that one has to draw the line somewhere, and I wanted to disincentivize any relation hierarchy building. This has a ton of pathological cases (a relation with no members, circular references, and the like) for relatively little value.