Synced video in photos app doesn't load in airplane mode

I've got a weird problem with Photos App, either on iPhone and iPad. I have all my photos on iCloud. On both my iPhone and iPad, I checked "Download and keep originals", in order to keep everything on the device. That is working correctly with photos, but not videos !

For example, if I take a short video with my iPhone, it appears a little bit later on the iPad thanks to iCloud synchronization. I can read the video on the iPad, but as soon as I turn airplane mode ON, I can't read it anymore.

This is not a storage problem as I have a lot, and I even reset completely the device, but still got the problem. Updating to Catalina and iOS 13 didn't fix the problem too.

Apple support admits the problem, but they are not able to solve it for months...

Does anyone experience the same problem or has an idea ?

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.