Additional features, settings, or issues not covered by specific Microsoft Teams categories
I just noticed this myself and wonder if I didn't have this problem before until now.
I, too, created a plan outside of Teams. In my case I did it through a Web browser (still don't understand why there isn't a desktop app for planner like there is for other activities). When I click on the drop-down carrot to link an existing plan, it won't drop down and show me choices. It seems stuck.
Looks like this has been going on for a very long long time.