I have checked Wowhead and Petopia and the area they say too look I have never seen a corpsefly. I have not only regularly checked the area they say, I have looked all over Maldraxus
Maldraxxus 65,70, if you look at the map it’s right between the Plaguefall dungeon entrance and the “House of Plagues” name (you can use Beast Tracker to found them easier)
They are not rare or anything and you should be able to found all the available Corpseflies colors in the area.
Can be found flying around the tops of mushrooms in the north-eastern section of the House of Plagues.
Haven’t looked for them myself but there might just not be a lot of them up at the same time, so perhaps just a matter of circling the area until one shows up.