I started getting the pre-roll ads for Critical Role when using Google Podcasts maybe a couple months ago. It seems like they're using a service which stitches ads in when the audio file is served so they can serve more up to date or regional ads. So if you had downloaded a bunch of CR at once months ago on Google Podcasts it might explain the behavior. Not sure why PocketCasts wouldn't have the same unless they're Caching episodes on PocketCast servers when they're first uploaded instead of downloading them from CR servers each time.

No first hand experience but I hear good things and Wikipedia has some info for further reading. https://en.m.wikipedia.org/wiki/Corsi–Rosenthal_Box#Efficacy