When the comparator skips, noise on Vcc is produced. This can be a small oscillation, a momentary dip or spike on Vcc. Even if you put a decoupling capacitor directly at micro's supply pins, there will be some rest interference on Vcc. This interference now is coupled directly to the inputs of comparator by the components Rp and Rq. This can cause an unsane feedback, making the comparator to oscillate! To guarantee, that supply noise of micro is not fed directly to inputs of comparator, points where Rp and Rq are connected to Vcc must be low pass filtered. This will prevent unsane feedback and mistriggerings.
As R21 is part of contaminated Vcc, because here a spiky current is produced, when output of comparator toggles, this noisy point should not be connected to that filter, that tries to keep Rq and Rp clean. Let R21 be connected to Vcc decoupling capacitor of micro.

Kai