The snapshot will use the resolution of the screen(s) where you are running your experience.
If you run the XP in Composer on a Full HD screen while testing your work, the screen shot will be 1920 x 1080
If you the the XP in Player while spread on 3 x 4K screens and take a snapshot of the scene, the snapshot will have the 3 x 4K resolution.
That’s what I have at least while testing on 3 HD screens.
Con you confirm you tested in these conditions?