With the changes to T-maps, Miasma chests can have ANY of the facets' maps. I know I've taken less than 10 Miasma chests to get all 6 facets represented before.
Now, getting a map IN THE CORPSE (not in a chest) on a Greater Mongbat, is odd. They are programmed to generate a chance for a level 1 paragon chest, with a "dummy" chance for a non-spawning level 0 being the basis for it. There shouldn't be stand-alone maps on Greater Mongbats AT ALL.
IF there have been, then it's possible that something in the T-map changes is causing what should be the phantom "level 0" map to increase to level 1 (As if the 0 was an error), and actually spawn. This might be a side-effect of changes meant to compensate for the existence of a few "Youthful" T-maps left over from before ML, as if those were done by a "Young" character, the resulting chest would contain a replacement Youthful map every time.