There is no problem with your camera in the sense that it is broken.
But your phone will use algorithms to "translate" camera pixel date to a jpg picture and use noise reduction and "overall" photo improvements.
The black bars are the result of "not perfect" calculations because your picture is "bright" in the bottom but dark in the top and your phone is trying to make the best out of it.
I am not sure which camera app you use but another camera app could give a different result.
But if you use Lumia Camera 5 you can save 5, 5+16 jpeg or 5+DNG.
If you would compare a 5mp a 16mp and a DNG picture you also will see differences.
A 5 mp picture is the result of the "most" calculations and for the 16 mp there are less calculations and for the DNG there are no calculations.
Furthermore not using auto but using manual camera settings and using a different focus point will give different results.
Thank you very much for your answer!
That photo in particular was taken with manual settings using Lumia camera classic, I also use Proshot and Lumia camera 5.
Afterword's I toke some 5MP + DNGs so I could compare them, the bars seem less visible on the DNG but by exaggerating exposure in post processing they became very apparent like here.
You're right, the algorithms do make it worse when translating the picture to a jpg file.
Am I expecting too much from a camera phone?