Yeah the image is 20KB for me. Upscaled by going to Document -> Resize Document. Use Lanczos 3.
Attached the image which I used. The file was open in Affinity photo and it was copied with making a selection and then copying and pasting.
I attached a video trying to reproduce it, but not able to fully reproduce it. I managed to generate two files (attached) which are 9.7MB before rasterisation and 1.4MB after. But the Gaussian blur worked fine. I imagine when I originally created that file I was making more layers and merging them to get that. (If it not clear, in the video I resized the image once, then undid it and pasted the image before resizing in case resizing the pasted image had any effect.)
affinity_photo_bug_follow_up.mov
Normal_map_example_-_Map.png.zip
trying_to_reproduce_1.afphoto
trying_to_reproduce_2.afphoto