I've had similar experiences in the last few days, and it never used to make those kinds of mistakes. It could hallucinate, but it never changed topics randomly or misunderstood prompts.

I haven't checked the API side, but I hope this reduction of capabilities is only happening on the ChatGPT Plus side of things.

It's upsetting. It's like the ChatGPT 4 we had a month ago never existed. Now it can't reason itself out of a paper bag.