I found another answer:
Activated Windows Device Not Listed in My Profile - Microsoft Community
Recently, Microsoft made changes that now allow only Surface devices to consistently appear under the "Registered Devices" section. If you have third-party devices (such as HP or Dell) it’s not an issue if they don’t appear. There is currently no fix for this behavior, and these devices may or may not show up.
If that's actually true, it means that the devices are missing "by design".