"اَلْباب مَكْسور في ٱلْطّابِق اَلْثّاني."
Translation:The door is broken on the second floor.
I'd say it is a badly sequenced sentence, both in Arabic and in English. When putting the "on the second floor" after the "broken", it qualifies (where the door is) "broken" - which doesn't make sense. The intent must have been to qualify which DOOR is broken, so in both languages the "on the second floor" should appear immediately after the "door", and the "broken" at the end of tge sentence.
I put this answer too at first. My inititial thought was that the option should have been allowed but on seeing the other examples it seems that when putting the adjective after a noun with an article (rather that after what would be an English verb to be), the adjective must also take an article, as you have proposed as the correct solution.