How often does this happen in practice and how does it happen? It's not like I google for a podcast each time I want to listen to the new episode and might get sucked into a fake search results with different ads. I add a feed to my podcast app and never touch it again (and I'm adding from a curated podcast catalogue, read: official podcasts only). Maybe if you were trolling for an ad-free feed you might find something but I've never seen what you are describing in the wild.