Tracking, updating, and organizing tasks within Microsoft Teams or integrated apps
That's the same issue I have, and I haven't found a fix for that yet. I *have* found recent documentation indicating that it's supposed to work, so I don't think it's a deprecated feature that's just "hanging around" for some reason. But still, I can't get my flagged email messages to show up in my Planner list in the "flagged messages" tab.