Bugged Waystone ? - Vaal Factory

I just experienced a weird waystone in a Vaal Factory Tier 7.

It was almost empty of any mob, I traveled the whole map with some monsters from time to time and one rare to kill in order to complete the map.

It was supposed to have 16 additional packs of undead but there was none.

Map is fully cleared and it says more than 50 monsters remaining.

Is it due to the Vaal Factory layout which don't allow some kind of monsters ?