MS Planner Plan - Add Admin

How can we add an admin to an existing Planner, lets say in a situation where the only owner is unavailable to add others for some reason. As a SharePoint, Teams , and Exchange Admin, we tried adding a user (same service account who is SPO, EXO, Teams admin) but when we open planner web https://planner.cloud.microsoft/ this Plan isn't listed. We only want to delete this plan and not the related Team or M365 group. 

Is this a limitation and there is no way an admin can add other users to a Plan?

Tried querying the plan via power shell and Graph Explorer but receiving unauthorised error even being an owner of the M365 group and Team that has this plan.