Maybe someone has already said this, but to me this is kind of a simple issue. Knytt stories lets you identify a level using 2 different tags. Since many of the tags are, in the views of many here, incompatible, it seems obvious that the tag that comes first is the primary tag of the level, and whatever tag comes second is a modifier of that tag.
So, if you are JUST doing an environmental level, you wouldn't even use the 2nd tag. If it was an environmental with a spot or two that could be considered maze or puzzle like, you don't just drop the environmental tag altogether, you modify it with the second tag. Likewise, if you have a challenge level with a lot of scenery, it makes a ton of since to have the modifying tag (the 2nd tag) be environmental. Even if this changes the meaning of the tags, it is the closest we can get in the system that currently exists and it seems very straight forward.