So I think this is the key for the issue of the saved items being wiped 
, or at least seems the common denominator when I get the issue.
If you add something to your basket and then sign in the site then thinks what you have in your basket and your saved list (which would be empty as you were not signed in) is then what should be in those places when your signed in. So it deletes and replaces the saved list.
This is obviously even easier to do as the site logs you out when you come of the tab so you have to re-log in more than you should which creates more chances to wipe your wishlist as you may have added something to your saved lidt without realising you were logged out by switching tabs for a brief period before then signing in. This I think then replaces your saved list with which ever items you just saved when logged out. (old site used to force you to log in to add to wishlist to avoid this issue me thinks 
)
I haven’t put this to the test as I have too much in my saved items but since I have been very careful with logging in and adding things to the basket I have not had anything wiped yet 