I have read some threads abut this problem on earlier versions of Lightroom and tried them without success. No matter if the videos come a Nikon Z6 ii or an iPhone 13 or any other source. Also, h.264 MKV videos are not even seen by Lightroom.
Thanks for reaching out. We are sorry for the trouble with Lightroom Classic. I'd love to help you figure this out.
Could you please share the version of the macOS on your computer? Has this started to happen since a recent upgrade of the macOS or since Lightroom Classic v11.3.1? Where are the original videos stored, on an internal drive or an external drive? Do these videos play in Quicktime or any other video players? Which variant of Apple Mac are you using, Intel-based or Apple Silicon M1 series?
Have you tried any troubleshooting steps?
Lightroom Classic uses Camera Raw Plugin for images & DynamicLink for handling video files.