4o was very sycophantic so was very willing to play along with and validate the users roleplay. OpenAI even noticed enough to talk about it in a blog: https://openai.com/index/sycophancy-in-gpt-4o/
I suspect that OpenAI knew that their product was addictive, potentially dialed up the addictiveness as a business strategy, and is playing dumb about the whole thing.
