AI unpredictability? Today, GPT did not obey a permanent instruction it had observed for months

I have a permanent instruction in my ChatGPT Plus preferences: "Write today's date at start of conversation". A simple task that the LLM always executed flawlessly in the last few months. So, I thought it was a given. No possible conflict with other instructions or system prompt or anything else. Nothing too complex that could compete for resources. Then, this morning, I started a new chat (a short spelling & grammar check prompt) and the reply had no date. I noticed it. GPT said "* You’re right. I should have included it at the start of the conversation. *" I asked why this happened.