Hmm, I think sometime in spring maybe. My wife tells me the same thing, she doesn't see those breaks. Not sure, I've had my Instagram account since 2010 I think, maybe like a reverse-grandfathering or just long-running A/B test?

I opened Instagram and scrolled now just to reproduce it again, and I saw one "normal" post before this "ad break" appeared for me: https://i.imgur.com/SS6kuKV.png