Carthographer Box - drop bug (?)

While clearing Zana's daily quest map (Strand) I encountered a blue carthographer box that had "1163 increased rarity" and "Explodes" mods (actually it was normal but I used an orb on it). When it opened it did not drop blue and rare maps, as I expected. Instead I got 7-8 normal maps, a divination card and a unique CHAINMAIL (not a map!).

Is this a bug or a feature? Should carthographer box drop non-maps?

Uniques come from a different item pool than the rest of the loot. So any box that gets the mod to drop a unique can drop any unique. Not limited to the type of box it is.