System Information
Operating system: Windows 10
Graphics card: nVidia GeForce GTX 980 Ti
Blender Version
Broken: 2.80 and newer
Worked: 2.79
Short description of error
In 2.79 we can draw individual pixels by setting the brush size to 1px, like in this example with drawing on 16x16px image:
Starting from 2.80 the drawing using 1px brush produces some extra pixels:
As you can see the antialiasing is disabled and it seems no other settings make any difference.