A bit more info: Initially I tested on 2 TV series that were on deck. With both of them, the next episode disappeared from on deck after metadata was refreshed. The files for these 2 series were still intact and I could see that the metadata had been updated. Trailers are also available that was not previously available. I then tested on a 3rd series and the on-deck thumbnail for that series also disappeared after updating the metadata.
I do have a fairly recent backup of the database, so I bit the bullet and updated metadata for all series. As previously stated, "on deck" was wiped.
I did not restart the server, but the thumbnails were missing both on the Home screen and the TV Shows screen.
These thumbnails are repopulating as we watch episodes, so the information is being pulled to the on deck segments of Home and TV Shows.