First time using 'internet' search.
Selected several results (20+) and saved to a custom list. A total of 9 were saved because I wasn't yet a subscriber to that service. I liked the idea once I found out search does more than I originally though. So I subbed. Exit NB.
Come back later to play with search. The original 9 were still in the custom list. Ran the same search again. All showed 'downloaded', probably because it thought I had 'successfully' saved them earlier in the day? I selected them again and chose to add to the list. It went through the motions and another 15 or so were added - the list even updated while it was already open. Then I add another group from a different location in the search results. Now all but one entry in the custom list are gone.
I deleted the custom list to start from scratch. Now none of them can be added. Perhaps because their status is downloaded. I can't find a Save Files To Custom List - Bypass Filters.