If a course has been built with only YouTube IDs, and these YouTube IDs are different than the one associated with the edX ID the transcripts will not render if the edX ID is added to the video unit. The example is that the MIT course 21W.789 uploaded all of their videos to a YouTube channel "ODL MITx" and build the course with these YouTube links. They also submitted the same videos to edX for processing and the edX media team had a YouTube channel "21W789" that the videos were pushed to along with S3 links. The course team did not use the YouTube IDs from the 21W789 channel but rather used the YouTube IDs from the "ODL MITx" channel. If the edX ID is added to the video unit corresponding to the the video but with a different YouTube ID the new YouTube ID is filled into the YouTube URL field in the video unit causing the transcript to no longer render as it is tied to the old YouTube ID. See youtube video showing the steps to recreate this issue.