This is due to Painter displaying the texture at its native power of two resolution, where the Pixel8r resolution can't scale up uniformly to match it. You can safely ignore the stretching.
For resolutions like this, follow the user guide and export out at full resolution, then downscale in any image editing application. No pixels will appear different than the preview in Painter, and you won't have any stretching anymore.
No timeline, but in the next version I'll be releasing a shader which fixes this problem.