This is a longstanding bug, and it’s partly YouTube’s fault. But it also appears that a small change could fix this issue on Voyager’s end.
Some, not all, but quite a lot, of videos will not show the thumbnail when browsing a community with large previews enabled.
My working theory:
The issue is the uploader didn’t make a dedicated thumbnail when they posted the video on YouTube. YouTube has to take a frame from the video itself and use that as the thumbnail on their website.
In Voyager, if a dedicated thumbnail was created, it shows up in all views. But if YouTube generated one from a video frame, it does not.
However, it does show all thumbnails when the Voyager previews are smaller.
For whatever reason, Voyager is willing to pull the YouTube-generated thumbnail in that view. Maybe it has to do with a resolution cut off, that generated thumbnails are usually from older, lower-resolution videos. But most all videos being uploaded today are in at least 1080p anyway. And a blurrry thumbnail would be preferable to the thumbnail meant to indicate a deleted video. I imagine it discourages people from clicking on them in the first place.
Is it possible to let the (potentially) smaller thumbnail through on large previews in Voyager?