Backed up videos won't play on the iPad they were backed up from
You have iCloud Photo Library'a memory saving feature activated, and either is your Internet extremely slow or you're experiencing a bug, to fix the bug restart your device.

To deactivate the memory saving feature in photos:
Settings->Photos &amp; camera -> Save and Store originals (or some synonym for it)
This requires you have enough free space on your device for all your pictures

Information from Apple's support page: https://support.apple.com/en-us/HT204264