I have noticed that the lists I create in my library account are often
deleted in much less than 365 days. In fact, sometimes, the list
disappears only to return a few days later with some books missing. I
guess my suggestion is that if this is an inconsistent mechanism,
better to get rid of it than end up with people losing information.
The Office staff hasn't experienced this problem, although we rarely use the "My List" feature. However, there were groans (as much as one can groan through email) when we mentioned My List to our colleagues in the Library's ILS department. Apparently this feature and its underlying technology has been a can of worms for some time now.
Good news, however: over the summer we will be testing the My Discoveries module for Lens, which provides for list creation and much more. By autumn quarter we hope to have a better alternative in place for list-makers.