Indeed I see it is not 1:1 and I see a lot of people having issues. I worked though the articles and they mostly describe on how to get things right in Teams, I personally am more interested in getting the Planner website to work right.
Is there a way I can see these hidden groups. Once I start deleting tabs in Teams, I loose track of Plans as I don't know where to find them in e.g. Planner.
And an update, it would look like also the naming influences the "default" plan to be displayed in Planner, when you add plans to Teams.
Once I added a plan with the same name as the Team, that plan became the one visible in Planner for the group. The others were still reachable via Teams tabs. The original plan that was associated with Planner for the team, became invisible (but still present).
Deleting the new "default" plan then, caused a new plan to be created as default for the group. Funny. Still the very first one was invisible.
So I headed to Graph Explorer to find out which Plans actually existed and used that to open them in Planner. From there on I could delete the inaccessible plans.
My conclusion, don't try to give a plan the same name as a team. Make sure you always have at least one task assigned to you (helps the graph api) and make sure you are the owner. That guarantees that you remain in control.