Currently testing a bit on Nova, and indeed what I am seeing is when you Mask Scent all by itself, no camo kit, it lasts 4minutes and 20 seconds with the Master Scout skill modifiers. However, if you use Mask Scent and then immidiately use a camo kit (Conceal) the "Aroma Scent" buff lasted 2 minutes, and when that two minutes expires it also removes the icon next to you character that you are "Mask Scent"ed as well as telling you that you are no longer concealed. In the combat log it doesn't actually say you are the "Mask Scent" is done, but it definitely removed the icon. It will then give you the message after the full 4:20 that Mask Scent has worn off, or when you Agro a mob, whichever happens first.

It seems to me that the "Aroma Scent" buff wearing off is removing the Mask Scent status as well. Will repost this in the bug reports.