Why?
Because it tries uploading it directly to the post, comment, or course page. Which is fine when it works. But when it doesn't, you lose your video. The app doesn't add your video to the photo library on your phone so you don't have a backup when things go bad.
Just use your favorite camera app instead and add it using the upload feature. It only takes a bit more time, but you'll have a backup.
this post was inspired by your situation 🙈