In the Media Info for these items, I assume that “Indexes sd” isn’t present?
Yes, Plex can generate video preview thumbnails for HEVC content.
It does so by extracting keyframes from the video. There are a few scenarios where unusually encoded videos can prevent this process from succeeding.
If you’d like to share a sample file with me, I can confirm or deny that it’s related to the file encoding.
Alternatively, you can try changing a hidden Plex setting. This may slow down all Video Preview Thumbnail generation, but it’s also more reliable.
GenerateBIFKeyframesOnly="0"
Instructions to add hidden settings:
Advanced, Hidden Server Settings | Plex Support
Here’s an alternative way to set the value (customize this for GenerateBIFKeyframesOnly="0"):
Big Media folder? Make smaller video preview thumbnails! - #6 by Volts
One related link & discussion; it includes links to others:
Video Preview Thumbnail Generation Fails for Some Items - #48 by Volts
