@martinkoenig - Board and Project permissions are at the user level, and not role, so you’ll need to make a permissions change to allow for the new person to see the projects:

More on project here → https://help.miro.com/hc/en-us/articles/360018262033-Projects

That being said, if you and Enterprise subscription, you can explore promoting the Team Admin to Company Admin and enabling Content Admin permissions - more on this here: https://help.miro.com/hc/en-us/articles/360012777280-Content-Admin-permissions-for-Company-Admins