I’ve been away for a bit and wanted to check back on this. I actually went into the previews folder and found that the preview videos themselves are the problem. When I try and play one I get a black screen, but the audio is there. The good part is that it means PhotoStructure is playing the video properly. However, it definitely seems like something is going wonky with the transcoding of the preview from a 4k video source (specifically iPhone).
Edit: I was finally able to do some additional troubleshooting as I was planning on wiping my library anyway. I moved over a few 4k files from my iPhone and set the library to not transcode, and had some interesting results. On Windows, Chrome would only play audio, and Edge would play audio and video, but the video was choppy. On Mac, Chrome only plays audio only, and Safari plays flawlessly. On iOS, Safari plays flawlessly. And, none of the videos play in the PhotoStructure app.
With the native 4k stream, Safari is clearly handling the videos no problem, but everything else kind of chokes. Is Apple doing something that weird with their iPhone files that it impacts playback and transcoding?