I have had the same issue and have found the error exists when selecting the "Broadcast Settings" when you do your project build selection in the very beginning. Do not use the "Broadcast Settings" seems to be the solution as Adobe has changed things there it would seem. If you are too deep in your project, you can select all clips in your timeline and then drag them to the "New Item" folder in the lowe right hand corner of your project panel. This will create a stereo timeline sequence by default it would seem. However, I was editing in 4K and when I did this the "New Item" created a 1080p timeline. So if you are in 1080 already then this is good solution, if not.. then just create a new project not using the Broadcast Settings would be the best practice it would seem. I hope there is another solution, but that is all I have been able to find at this time.
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