What the achievement asks
Cleanse seven Consuming Maws in Uldum and the Vale of Eternal Blossoms during Black Empire assaults. Five points, account-wide, both factions.
The description is misleading
It says "every unique Consuming Maw objective", which reads as seven different locations. It is not.
The same maw counts again and again. Each spawn carries a hidden daily quest that stops you repeating it within a day, and once that resets, the same maw pays out again. Seven cleanses is seven cleanses, not a tour of seven sites.
You can tell whether a maw will pay you: stand near it and look at your minimap. A blue arrow pointing to it means the credit is available. No arrow means you have already done that one today.
They only exist where N'Zoth is attacking
Maws spawn during a Black Empire assault and nowhere else. Going to a known location during an Amathet or Aqir assault finds nothing at all, because the assault running there is the wrong one.
Their timers are independent of each other, so it is normal to find four up at once or none. When several are up, all of them count.
Two things that look like failure and are not
Being thrown out mid-cleanse. If other players are inside, a maw can despawn without paying you. You are not locked out: wait a few minutes for it to come back and go in again.
Alts. The daily lock is per character while the achievement itself is shared across your account, so a second character can cleanse the same maw on the same day and the credit still lands. Anyone in a hurry can finish this in a single Black Empire assault by cycling through a few characters.
The same assault window
Everything else in patch 8.3 that is gated on this assault should be done in the same visit: It's Not A Tumor! wants a hundred tumors and Combating the Corruption wants ten kinds of corrupted creature, both only here.