I don't think it's the case because static dir is relative to your app.py.
Take a look in rendered source from your browser.
Compare what you wrote with what you get in there.
You should see the same: <link rel="stylesheet" type="text/css" href="/static/styles.css" />
Another thing you should see is your browser's cache. I'm facing some issues with it.
Try turn off browser's cache.
I hope it helps.