How to disable dithering with Palettized PNG exports?

[M-Tee]

I'm trying to reduce color count in images with large, flat areas of color. I reckon there's a setting I'm not finding somewhere.

This is what I'm getting in Affinity Photo:

 

This is what I'm expecting:
in Photoshop


or this, in Photopea

 

[lacerto]

Affinity apps are very elementary with indexed (paletted) images, and can only export some limited formats; they do not support opening / editing indexed images but open them as RGB images. Color reduction without dithering is not supported, either, so you would need to do this using other software, like Photoshop. As for free image editors, at least GIMP can do this, including using user-defined palettes.