URL paths over https wouldn't be transparent to the ISP though, would they?

They would not - GP was probably bringing up something not directly relevant, but still related. (they should have clarified though)

What can I add? I know the domain could be found. But how did Google index a page that is example.com/$MD5.html - guessing 128bit numbers is hard.

I have visited that page from a signed-in Chrome profile.

Erm sorry when I said GP I meant of my comment - that is, phoghed's irrelevant comment about ISPs collecting data.

The information you mentioned is relevant. Unfortunately, it could be either Google/Chrome, or the LLM service you're using for development is misusing your data.

That assumes it was using it from the get go and every time, which might not be a safe assumption.