I think the number of issues is directly proportional with a library or framework's popularity and and the platforms it targets. Let's not ignore the fact that Flutter is an unprecedented effort. I can't think of a single SDK that tries to provide a solution for mobile, web and desktop in one go.
Take for instance Angular that targets web and mobile. 2.7K issues currently and this is a 5 year old framework. And that's not even enough because technically if you want to build mobile apps, you'll need something like Ionic which comes with its 650 issues. Then if you want to build desktop apps, you will take on Electron's 1.3K issues. That's 4650 issues right there.
I think for its age Flutter is actually doing quite well in terms of issue count. A lot of those are also just questions by people who submit issues instead of asking on StackOverflow or doing some half-decent googling before asking a question.
Another thing that I wish people would do less is moan about the bugs instead of contributing with a fix. It's an open-source project for a reason.