You can enable or disable dithering. If enabled, error diffusion is used. Palette is generated and in general, all 256 colors should be used. If the saved image contains much less colors, it is a bug. In that case, please send the problematic file in some other format (so that the colors are not already lost) to info@rw-designer.com and I'll examine it.
GIF Animation pallete optimization options
Found that RWPaint has no 8-bit pallete optimizations options - no methods of dithering, and when I'm saving some 256 animation to .gif it loses much more colors than original one...