As of February 24th, 2020, our engineering team isolated an issue where videos uploaded to our iOS app weren't respecting edits made in the Photos app and saved to the original video file.
This bug is limited to .mp4 files specifically--for example, screen recordings. If you make an edit to an mp4 file in Photos.app, such as trimming the video's time, then use the Save Video option and attempt to upload the edited video to Discord, the original video will be uploaded instead.
This issue will be fixed in our forthcoming stable release on the App Store. For now, the best workaround would be to save your edited mp4s using the Save Video as New Clip option, then upload your edited clip rather than the edited original.