ChatGPT free user plan - GPT-4o sometimes start reasoning by itself ?

Hello everyone, I would like to ask you if you noticed something different in the recent days with GPT-4o for free user plan. I happened to see GPT-4o starting reasoning by its own for complex queries (such as code questions or structured questions), throught the classical "I'm thinking" bar before get answered. The point is, ChatGPT didn't switch from GPT-4o to o4-mini in order to reason, but the reasoning process seems to be conducted from GPT-4o by itself and just whenever it decides to do it. Now, this happened to me but I didn't find any news online, did somebody know anything about this fact? Ty