Right now, it is not possible to list the plans of a private group if I am not a member even if I am owner of said group. To me that does not make sense and as a owner, I should at least have the same rights as a member and I should have access. All our groups were made providing only the owners in the owners list and they are not in the members list so we have to manually add them to the members list as well before querying the plans.
Here is my post in the MS Q&A explaining the issue : https://docs.microsoft.com/en-us/answers/questions/795788/cannot-list-planner-plans-for-a-private-groupteam.html?childToView=800441#comment-800441