Well that reason is out the Window kinda Mala.

Fomor RNG mobs's ranged Attacks are not 100% Shadow-Taking. They follow the same rules/patterns as normal attacks.

My guess is that, Back when, They probably register "Ranged Attacks" from Mobs as TP moves, and TP moves always take a shadow, hit or miss. but when they released RNG-Fomors, Who probably mimic players closer than we think, They follow the same patterns as PCs, Ranged Attacks not taking shadows.

I suck at explaining but i hope you see what i'm trying to say. Theres inconsistencies. RNG-Beastmen attacks are 100% Taking Shadows, while RNG fomors attacks Do not.

(Then again, When you think of it from Sleep Bolt point of View, Fomors would resist them to high hell LoL.)