Krita doesn't dither by default. I recommend working with 16 bits integer at all times within Krita for normal maps and using your engine to do the conversion to 8-bit. That way, the color management doesn't mess with the normal data. If you need to add some dithering on top of your normal map to break up banding that…
This is a complicated subject but essentially there isn't any way that you can reduce 32bit float to 8bit without some kind of dithering unfortunately. The best you can probably do is to use hard edges around your UV islands so the normal map is less colourful so it compresses better (vs one smoothing group for everything)…