Video recorded on newer iPhones or iPads are usually saved in HEVC (High Efficiency Video Coding) format to take up less space on the device. This format of video has limited support on Subly and so may lead to some discolouration of the video. If you are wanting to burn subtitles onto your video, we recommend first exporting your HEVC to a more compatible H.264 format before uploading to Subly. Please see details from Apple https://support.apple.com/en-us/HT207022
If you still think this is a Subly issue, contact us directly.