This seems like a very “niche” issue, but my concern is that
a) It works on Unity 5.02
b) It broke in Unity 5.1
c) It works on iOS 8 with il2cpp, works on Editor and other platforms like WebGL (and Mono backend as well)
So to me it is a symptom of an il2cpp translation issue that occurs only on iOS 7, and it was introduced sometime in the 5.1 cycle. Identifying it might help ensure more stability for the il2cpp backend.