A video walkthrough of the quest "Rat Problem" available in chapter 3 of FF7 Remake!
During my normal difficulty playthrough, I had to leave and return to the area to get the Doomrats to appear. During my hard playthrough they showed up immediate upon defeating the Wererats. I don't know what the difference was in what I did, so just leave the area and return later if they don't show up after defeating the Wererats.
Thanks for watching!