Did you noticed, that if you get task to kill triglavians and drones in Abyss, you will see nothing but drifters in few first filaments? That started with current event.
I hope CCP will change that back to normal, because diversity of opponent is what makes Abyss space particulary interesting PvE. And facing same waves in every single room you get makes Abyss as dull as null anomaly grinding.
Every instance is unique and and largely random. There is a large pool of “room” types and only three are selected. It is sheer coincidence that you are getting instances that have rooms without the rats you are looking for.
I definitely noticed that the tendency of the RNG to make this take much longer than necessary felt borderline-intentional. Almost every time that I needed to kill a large group of something for the highest-rewarding task, there were barely any such enemies to be found.