The Catch Up (Q) mechanic for the rats doesn’t respect vertical distance.
In some spots, such as the end of Righteous Stand Section 1 (tower/stairwell) or Against the Grain Section 1 (the big barn) the vertical distance can be quite huge while the horizontal distance is minimal, leading to the Catch Up mechanic being unavailable.
This feels pretty bad for the rats, especially those already spawned, since there is no way to get to the heroes at all.