It did it again today, but I noticed that there was an unresolved recording conflict that hadn't been flagged by my box. The series link had been silently removed. I only saw the conflict when I added it back.
So, is this normal, to remove a series link if the box spots a conflict? I expected the link to just remain dormant.
Anyway, solved by removing Wolf Hall. The books are dreadful, anyway, and I guess the series will be no better.