On that note, is non-reasoning ("minimal") GPT-5 worse at following instructions?

I'll tell it don't use numbers or bullet points and it just ignores that. Unless I scold it, then it complies.

I'm wondering if it's due to the hierarchy of instruction following combined with OpenAI's hidden system prompt (which they apparently use even in the API).

Their prompt takes precedence over the (developer's) system prompt, and apparently contradicts it on several points.

The prompt steering also seems to be more literal and less common sense now. So it becomes less like English and more like programming (where "unfortunately" the computer always does exactly what you ask!)