Last sponsor name was for 5 years, current is for 4 years, it changes, so hard to remember. Common name is more universal and permanent. Even on the sponsor's website they refer to it also as "Stadion Poznań".

So previous version with added official_name, similar as on Wikipedia (sponsor name is listed below common name), was fine.

Alternatively we can choose between: "Stadion Miejski" or "Stadion Poznań", or back to "Stadion Miejski w Poznaniu".