I have figured out the resizing problem! Before resizing, it is VERY important to Rasterise your image (Right-click, Rasterise). And then once resized, I recommend raterising once more. This completely fixes this resizing problem. Using the various options makes fine improvements, but even with the default, this rasterising approach works.
I'm no programmer, but it seems that Afinity "holds" the image in memory, until such time that you "lock it in", using rasterise, or exporting to a final file. By adding a resize in there clearly seems to confuse Afinity, as it seems to resize using some sort of proxy screen image, that is not actually the actual hi-res image. Anyway, that is how I rationalise it to myself.
Happy once more :-)