When I export the same image from Affinity Designer, there is a very clear difference in terms of brightness.
To test it, I created a simple black to white gradient.
Using the three different Colour Formats (with the same Colour Profile: sRGB IEC61966-2.1 (Linear)) in the "Document Setup..." panel, I exported three images.
Here are RGB/8, RGB/16, and RGB/32 (HDR) respectively:
Also here are the histograms for the three images, put through Affinity Photo:
I am a self learning graphic designer so if this is intentional then I'm sorry, but shouldn't the images be virtually looking the same, save for the "banding" and the noises from PNG compression?