I think ChatGPT's answer here is defensible. You're asking it a pretty formal question but it's giving you an informal answer, which a reasonable default. It's

I think ChatGPT's answer here is defensible. You're asking it a pretty formal question but it's giving you an informal answer, which a reasonable default. It's not "assuming spherical cows", which is to say it is not assuming your question is completely self-contained. If you tell it that it's a formal logic question it gets it correct. (I checked this on GPT-3.5) In a real world situation, your conditional plans are not sufficient evidence to determine if it actually rained. And it explains the reason – there could be extenuating circumstances you haven't told it about. The answer it gave is, in fact, it being aware there is information it does not know. It could be better about inferring your intent, but that isn't the same kind of failure.