Black video screen with working audio on an exported video is usually caused by your iPhone or iPad not supporting the video resolution of the source files linked to your session. The files might play back fine, but the video transcoding will not work. For example iPhone 13 Pro can currently (2023) export video with source files in 4K resolution, but not with 5K resolution.

To fix the issue you should try the following:

• Record future videos with lower resolution (for example in 4K for iPhone 13 Pro)
• Downscale your high-resolution source videos to lower resolution: https://racechrono.com/article/faq/how-to-downscale-my-source-video-files