✳ THUMBNAIL DIAGNOSE
Diagnose: thumbnail / cover image is rejected or cropped wrong
Dynamic diagnostic state for this problem. Choose a file to run the diagnostic tree; the page rerenders with ranked causes and recommended fixes.
Direct answer
Each destination has its own thumbnail / cover size and aspect. The filename and even the source dimensions are not enough; the placement preview (feed card, profile grid, share card) is what decides the framing. Verify the placement and export a cover that respects that crop.
Run the diagnostic tree
The tree combines this problem's static cause ranking with the values the diagnostic engine reads from your file.