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The problem here lies with the app itself, just as Instagram, it compresses images so that it fits in 1080 pixels (of height if not mistaken) l too have a Mi8 6/128 and whenever I upload a WA status or IG story it gets compressed and effed' up. My advice is to either try recording in portrait 1080p or using a third party app to crop or resize the video to fit the limit of these apps. As far as I know, these limits are to make sure the video is playable in most phones and that it doesn't consume too much bandwidth to upload (as it takes time to do so).. If I have mistaken anywhere here and someone else has other thoughts and inputs to help this lad, be sure to comment.