It just happened to me today. Shocking a bit.
It's nothing more than a glitch in iCloud.com system. Might be related to the poor connection ? Just speculating.
Here is how I resolved it.
If you still have the device:
Sign the device in again to the iCloud (you should notice the device name changed in iCloud.com + LogOut/LogIn required) Then, turn off all the iCloud services on the old device again with "Find my iPad" at the end. Sign out completely. Return to iCloud.com + LogOut/LogIn = you shouldn't see the device anymore.
If you do however - repeat this step.
If you haven't your device anymore:
Then the best thing is to wait 2 - 3 months for iCloud.com to refresh services. Always make sure you are correctly logged off / signed off before you sell the device. Nowadays technology is much more private than it was few years ago… People still tent to forget about it.
If you have a number next to your new device's name after you successfully removed your old one:
You will have to sign new device in again - it will bring back the correct name. Before you do that - be aware what you have synchronised with iCloud already to make sure you won't loose anything.
Do I have to remind you of backing up your devices before doing anything related to the possible files missing?
Have a great day!