Setup: one video item in Playlist. Single Video track with playlist item. Save. Remove Video track. (At this point the Timeline option is still available in the Export pane.)
Reproduce the bug: Remove the single video item in the Playlist. The Timeline option is no longer available in the Export pane.
If you add other video items to the Playlist, create Video tracks, etc… the Timeline option is never re-enabled in the Export From dropdown list in the Export Help pane. The only way to restore it is to quit and restart Shotcut.