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.
How long does a problem have to exist in Microsoft Answers before Microsoft replies to the problem? I didn't have a problem before, but now I'm one of several people who are experiencing this same problem. Don't call this website Answers.microsoft.com if
Microsoft is not willing to provide an answer. If you keep this up you could lose a lot of people to Zoho or another platform.