Fwiw I don't think pocket casts is doing that, but rather your podcasts themselves. A lot of podcasts use dynamic ad insertion that allow them to inject ads into the podcast at download time as opposed to them being baked into the episode. It lets the hosts have relevant ads in their entire backlog of episodes rather than just ones released during the campaign window.

I love pocket casts, but their recent price hikes are wild. AntennaPod is a nice open source alternative that I've been enjoying the last few weeks.