Hi Sean,
It's inner software in my company. It was created across TGA documentation and works well with TGA files exported from Gimp and Photoshop.
In other viewers it works fine (Photoshop, Gimp, Blender, MacOS default viewer, etc.).
I can acquire request for modification our soft but I think that better for Affinity is support TAG format correctly.
Example in attachments:
demo1.tga Header(hex) 00 00 02 00 00 00 00 00 00 00 00 00 31 00 31 00 20 20 (Affinity Photo)
demo2.tga Header(hex) 00 00 02 00 00 00 00 00 00 00 00 00 31 00 31 00 20 08 (Photoshop, no RLE compression)
demo3.tga Header(hex) 00 00 02 00 00 00 00 00 00 00 31 00 31 00 31 00 20 28 (Gimp, no RLE compression, origin top-left)
demo4.tga Header(hex) 00 00 02 00 00 00 00 00 00 00 00 00 31 00 31 00 20 08 (Gimp, no RLE compression, origin bottom-left)
As you see last 4 bits (0x0F) have information about alpha channel size (8).
demo4.tga
demo3.tga
demo2.tga
demo1.tga