I just confirmed it also. Photoshop CS6, Light Room 6, and PaintShop Pro X8 all display the image about the same size as each other regardless of Windows Display scaling. They seem to display an image pixel for pixel to match the monitor's hardware.
Light Room and PaintShop Pro definitely scale up their interface elements as they should, but the work space stays matched to the monitor's pixels.
Affinity Photo seems to be scaling it's work space along with the interface elements if Windows Display Scaling is above 100%.
Seems like this should be classified as a bug. I would think most people would want to see all the pixels that the display can create to make use of a 4K display. I guess the exception would be if Affinity is still making use of all the pixels in the display, even if we zoom out to 50%.
In other words, if Windows scaling is set to 200% and I open a 1024x768 image and zoom to 50%, will I still be seeing all 1024x768 pixels or will Affinity Photo scale that down to 512x384 and let Windows scale it up?