Two rulers can be expected to share reverses on coins struck at the same time in a provincial mint. It is not a 'mule' mistake but just the way they did things out there.
I do not have it but recall a posting on another forum showing two different cities in this region using the same obverse die with different reverses. The was I see that is that one mint made coins for both cities rather like the American Banknote Co, printed paper money for more than one country as a contractor. I have no idea where the mint for Marcianopolis was located but it is not safe to assume all coins of all cities were struck in that city.
For comparison see my Septimius from Anchialus, Thrace shown below. These may not be die matches but they do show that the type was popular in the region at the time.

I do not have it but recall a posting on another forum showing two different cities in this region using the same obverse die with different reverses. The was I see that is that one mint made coins for both cities rather like the American Banknote Co, printed paper money for more than one country as a contractor. I have no idea where the mint for Marcianopolis was located but it is not safe to assume all coins of all cities were struck in that city.
For comparison see my Septimius from Anchialus, Thrace shown below. These may not be die matches but they do show that the type was popular in the region at the time.




















