I remember encountering this same thing about 2 years ago while I was still frequenting the Store and was able to consistently fix it by trying to edit my wishlist shortly after logging in.
At first I would log out then immediately log back in each time I visited the Store, then edit my wishlist if necessary before going about my other browsing plans, but after some time I just started logging out when I was finished. If ever my browsing was taking a while and required editing my wishlist later on, like checking a longer string of Make Me an Offer deals, I would log out and back in before dealing with my wishlist then continue browsing.
Since I started I haven't had trouble editing my wishlist.