I don't question the existence of screen lag. I've observed it often, and drops in transmission, too.See an example of vídeo lag. Isso in Portuguese. But the left image is the real one, and the rught onde is the screen app
The drone descended because the pilot pulled the left stick back. He may not have had sight of the drone, but being aware of a lagging screen image and being over it was unwise to command the drone to descend.