I am not sure either what you are seeing. Perhaps you should post a screenshot as this might be something specific to your monitor profile or monitor. Anyway, there is one thing that could be causing this. Contrary to Photoshop, Lightroom does not dither when converting from the 16-bit source space (linear prophotoRGB) into 8-bit sRGB. This can in principle cause the appearance of banding, which seems to be what you are describing. They were going to fix this but it hasn't happened yet. Another possible cause for such banding is out-of-gamut problems, where the image is outside of the sRGB gamut, but still inside the monitor gamut. In that case, you will see anormal image inside Lightroom, but a clipped and banded image in sRGB. That usually only happens when you drive the saturation or vibrance too hard.
ages ago(permalink)