Photos doesn't read H265 video

Hmmm,

Using HEIF or HEVC media on Apple devices

Upgrade to iOS 11 or later or macOS High Sierra or later to view, edit, or duplicate HEIF or HEVC media captured with an iOS device.

iOS 11 and macOS High Sierra introduced support for these new, industry-standard media formats:

HEIF (High Efficiency Image File Format) for photos

HEVC (High Efficiency Video Coding), also known as H.265, for videos

HEIF and HEVC offer better compression than JPEG and H.264, so they use less storage space on your devices and iCloud Photos, while preserving the same visual quality.

To fully view, edit, or duplicate HEIF and HEVC media on your device, upgrade to the latest version of iOS 11 or later or macOS High Sierra or later.

https://support.apple.com/en-us/HT207022

FWIW, H265 is just a video codec. Depending on how the video in question was formatted it may cause an issue. For example if the audio codec that was used is incompatible. Or how the video was packaged. What is the source of the video?

Does QuickTime Player play them?

Interesting. If QTP won't play them, Photos won't. I suppose you could try downloading the High Sierra combo update which may replace whatever is causing the problem.

https://support.apple.com/kb/DL1970?viewlocale=en_US&locale=en_US

I do know there were security and supplemental updates for High Sierra. Have you installed those?

You could also try reinstalling High Sierra.

I see. Well, I no longer have High Sierra installed on anything so I can't check High Sierra issues. I do have H265 videos in Photos on Catalina. What extension does the file or files in question have? Mine are mp4s.

You're welcome.