Mid-2010 Mac Pro, macOS Sierra 10.12.1
Bookpedia 5.5.0
(Probably happens in DVDpedia as well, not tested.)
While viewing the library, click twice on one of the fields (for example: Series) so that the blue highlight appears around the field indicating it is in edit mode. Now click on one of the other lists (a smart collection, the borrowed list, etc.) and then click back to Library and located the record you were editing. Notice that the Series field (or whatever field you chose) is now blank. Clicking twice in that field will bring the value back but it now appears to be confused as to whether or not it is in edit mode as the blue outline is gone. If you click away from that field, the blue outline will appear but it is not in edit mode anymore. Also, continuing to use the application in this state (using the search field, double-clicking to edit items, etc.) will eventually cause the library's scroll bar to disappear!
Restarting the app corrects the problem.