Favourites are unable to disappear on their own as there is no current functionality for that.
A number of things may have happened:
Favourites are tied to the user not the account, they have logged into a different user on that account that doesn’t have the favourites saved
The products no longer exist on the website
User has accidently deleted favourites
if you wish to download a Buyer Favourites file and a Buyer User Favourites file - you could look in these for the missing Favourites, if they are still stored against a different login. (see below example of another instance of favourites going missing)
You can Download blank Header Files for each of the Favourites, and re-upload them, see below for downloading blank headers:
NOTE: The Buyer Favourites file, when uploaded get assigned to all Buyer Users (Logins) of the Buyer (Customer).
For upload:
