It seems like the position of the sun does make a differnence in how the scene looks.
the sun is only a little bit above the cube
the sun is 13 units higher above the cube than before.
As far as I know there should be no difference, but the ground is clearly brighter in the picture where the sun is closer. I used the “Uncharted” Tonemap in both examples as I think it makes the difference more obvious.
Is this a bug?
Concerning sun lamps, from what I understand position doesn’t matter. It’s the angle that changes its effect. Sun lamps general simulate a light source infinite distance away.
Edit: So you observations are surprising! 
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Looks like a bug! Did you make any changes in the Render Path properties? I tested a default cube here but that seems to be ok for some reason.
Oh yeah I have Voxel GI turned on, and if I turn it off, this bug disappears.
So will this be fixed in the next build? I would be happy if yes, but no presure you are already doing more for his community than any other developer I have ever heard of.
Yes, already fixed in git, will be included in upcoming build. 
Thanks for reporting the bug!