My pilea is developing large dark patches. No evidence of mites or root rot

Question

This happened practically overnight on two lower leaves only. I wait for soil to dry between waterings. It's been happy for the few months I've had it growing babies etc. I checked the roots and they seem fine. The patches seem to be starting at the centre of the lower leaves and spreading outward. I want to stop whatever this is from spreading to my, otherwise healthy, plant.