I confirm and bug report done : https://github.com/ubports/messaging-app/issues/192

@HendriXXX Thanks, your video helped a lot!

If I rotate the phone to landscape leftwise or rightwise -- both come out fine. However, it's only the rightwise rotated landscape video that was upsidedown when re-encoded from .mp4 to .mkv. Solved!

The issue seems fixed now.