Unwrapped object was filled out with gradient color. As illustrated in below image, some pixels in UV/Texture view on Seamed edges boundaries are not filled with expected color. It looks like edge has to cross more than a half of the pixel to be colorized.
Is there a way to force all pixels crossed by Seam edge to be colorized properly?
Found a solution. Navigate to Texture Paint for given object and open Options, modify Bleed property, which defines how many pixels will be colorized in UV texture outside Seamed edges. Default is 2px.