Sorry to hear about this, @BeanCounter! You’ve done a great job to factor out many variables through testing and comparison. Rebooting the router would have certainly resolved any communication concerns and, as you stated, the Camera itself seems to be producing these snapshots no problem.
The best next step will be to see what other differences might exist between Android mobile devices. I recommend ensuring vpn is disabled, as well as checking for any apps that might conflict with the Ring app. If possible, try logging in to another mobile device entirely to see if these snapshots appear as intended. Feel free to let us know how this goes! 