I have a large project which I have been pre-rendering as I've been working. Total render time is in excess of 20 hours. All of the black bars which indicate a pre-render is available are visible and I can navigate to the actual pre-rendered files (AKA They exist!)
The only settings I can use are the 'XAVC/XAVC S' mp4 settings in which there are 3 options which match my project. This is also how I pre-rendered the video. Selecting any of these options does not use the pre-rendered files.