I believe that Jan had it right in his comment - the problem was the distortion caused by the wide angle, resulting in the horizon at the left and right 'drooping' quite significantly.
It is possible to correct such distortion - I made an initial attempt in this example, which is not perfect (the result would be better working with the original), but it gives an impression:
Now that horizon is a lot straighter (and could be improved further, as the middle is still slightly higher than out at the edges of the shot). I think that would then resolve the issue for you. However, it would be interesting to know whether this would be considered an unacceptable 'manipulation' of the image.
Let me know if I can help some more.
All the best.