They've definitely changed something about the models, and it is in their interests to do so, both to create a low-latency experience, but most importantly, to save money.
While GPT-4 is still workable, GPT-3.5 flatly refuses requests these days, claiming that as an "AI language model" it couldn't help me write code.
Usually trying to regenerate a response works fine in these cases. However, claiming that the RLHF and subsequent fine tuning isn't having any effect is a bit dishonest on the part of OpenAI.
While GPT-4 is still workable, GPT-3.5 flatly refuses requests these days, claiming that as an "AI language model" it couldn't help me write code.