Hi fisherman,
This is definitely a compression corner case. Both figuratively and literally.
I found that the normal map you sent over is non-power-of-two and our resampling seems to have a small precision problem, causing discontinuities which are also visible when using lossless or no compression. I'll get this fixed.
Adding to that, our lossy compressor can't handle hard normal map edges that well. I'm going to patch the current dev version to allow different compression types per layer, so we can optionally use Lossless only on the normal layer. We'll have to improve the Lossy compressor regardless, but it'll take a bit longer.
If you're interested we can send over an updated trial once these change are in.
--- edit
Just to clarify:
Even in the current version, if you switch to power-of-two textures and use Lossy, at max or near-max quality (95-100) only in the Normal Map layer, you get more acceptable good results and better compression rates than Lossless.
![Image](http://i.imgur.com/OAc2f6d.jpg)
We can do better, however, and we will.