Unable to load video error after iphone migration

Greetings thatuseristaken, 

You mentioned that these videos are downloaded from external sources. Have you already tried redownloading the video from that source? Which app are you attempting to view these videos in? 

You also noted that you don’t sync data with iCloud. Have you confirmed that in Settings > your name > Photos this is disabled? We ask this because you mentioned about optimizing storage and when using iCloud Photos there is an option to optimize and store full resolution photos in iCloud. Manage your photo and video storage

With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.

Thanks. 

Thanks for replying, thatuseristaken, and that additional info, it really helps! At this point, please contact Apple directly for further investigation into this: Apple Support

Have a great day and thanks for using the Apple Support Communities!