Synced video in photos app doesn't load in airplane mode
Apple Community says this: 

Double click your home button and clear all open apps by swiping up. Then let’s go to settings>iTunes and app store and tap your Apple ID and sign out. Restart your device and then sign back in. Once done make sure wifi is turned on and go to the downloaded video application and make sure your downloaded content appears. Then go to airplane mode and check if the content still plays. Then clear app again and relaunch and test whilst still in airplane mode.