Setting Krita up to make a similar brushmark in the linear gamma sRGB color space, the same posterized edges appear. Using my modified version of an older version of GIMP, the same posterized edges appear though perhaps not as extreme - brush settings are very different in the two programs so I just made changes to the default Krita brush tip settings until it had a similar fall-off from center to edge, compared to the brush we’re using in GIMP.
Here’s a screenshot of the Krita brush marks at 200% zoom:
The weird thing is that there are no posterized edges in GIMP when drawing a radial gradient from white to black with the same radius as the brush stroke radius, even if the layer is subsequently modified with Curves or Levels to look more like the brush mark. But as soon as the radial gradient layer is turned into a mask and added to the layer, the posterized edges appear.
I’m working at 32-bit floating point in both GIMP and Krita. So I’m wondering if possibly the mask and/or the brush stroke and/or the alpha channel is somehow at 8-bit precision instead of 32-bit floating point precision in one or both of the programs. I’ll try asking around to find out, unless maybe someone reading this thread already knows? @Boudewijn_Rempt ? @Jehan ? @Americo ?