Map completion with rare monster still alive

Just had a map complete with a rare monster still alive in the map, that was not marked with a skull icon.

Name of rare still alive: "Tempestbringer the Faceshredder"
Monster type: Burrower

Thoughts on the cause: "Area contains 11 additional packs of Faridun Monsters" was on the map. It may have been a rare from one of these packs. I'm not sure if this should be considered a bug if that is the case, but figured I'd bring it to your attention.