Identify the missing preview rather than the video
A video can play normally while dragging the seek bar shows no preview images. Record those observations separately. The absence of a thumbnail is not, by itself, evidence that the programme cannot be decoded or that the stream has lost resolution.
Read the server and client roles
Jellyfin introduced Trickplay timeline previews in 10.9 and explicitly qualified them by client support. Its 12.0 release notes describe further generation and reuse fixes, including existing preview files and problematic source timestamps. A server supporting generation therefore does not establish that every client presents those images in the same interface. Likewise, a familiar client control is not proof that preview data exists for every library item.
Compare one prepared item on the intended client
For example, an administrator can identify a lawful file with preview generation completed, then inspect its seek behaviour on a client documented to support the feature. Keep that file constant when comparing another client. If only the preview differs while playback works, report the client and preview state rather than changing the video codec or replacing the file. Avoid repeatedly regenerating an entire large library before checking one known item. Include the server and client versions in the report; an old screenshot showing a preview on another app is not a guarantee for the current combination.
Questions, answered
Does server support prove every client can display timeline previews?
No. Jellyfin’s feature announcement explicitly requires client support.