Are you certain it was Spotify that did that? Unless you downloaded the episode in 2018, it could have easily been the Conan podcast. Podcast episodes aren't immutable, they're just a url. I've seen podcasts that update the ads in their back catalogue to whoever is paying them at the time the episode is downloaded.

I've also noticed some location specific ads, which isn't that surprising if you think about. There's nothing stopping them from serving you a different mp3 file depending on your IP geolocation.