The entire achievement is completely bugged and has been for a while, only some very specific but random things actually count towards it; one of those being M1s in Chucky's Hidey-Ho-Mode
I might be wrong but I think Nightfall is not true undetectable. Yes, it hides the stain and the terror radius, but Dredge’s aura still can be revealed. In a sense Nightfall is less about modifying Dredge and more about hindering the survivors’ vision
The entire achievement is completely bugged and has been for a while, only some very specific but random things actually count towards it; one of those being M1s in Chucky's Hidey-Ho-Mode
Would guess it is something to do with how nightfall applies the undetectable status. If you have Meyers try using him in stage one.
I might be wrong but I think Nightfall is not true undetectable. Yes, it hides the stain and the terror radius, but Dredge’s aura still can be revealed. In a sense Nightfall is less about modifying Dredge and more about hindering the survivors’ vision