I agree with you entirely about the issues with the docs, there is no API reference, and Tiangolo has rejected PRs with automated documentation before. I by no means think the project is perfect, but I think it is a better starting point for most small projects. I just don't see any advantage to Flask at this point unless you specifically want to use one of tiny handful of Flask-X packages that are actually worthwhile.
I wanted to add django but their issues aren't on github as they are using their own system. IMO an external issue tracker lead to a higher barrier to create new issues, so this isn't not necessary fair.
Also fastapi allows people to ask questions with the issues. On the 1000+ open issues, 826 are tagged as question.
It's hard to find a fair metric. Open/close ratio rewards older project with fluctuating/stable popularity. Fastapi popularity is still growing[0], so having some difficulties to work on issues is expected.
