I've been chasing a really weird bug for a couple days now.
When adding items to a wishlist the most recently added item is replaced by the new item. This happens even when all local modules are disabled.
The most recently added item is simply removed from the database table, and the new one inserted. Sometimes I can get more than one item onto the list, but it's very rare.
Others have had similar issues (https://stackoverflow.com/questions/28452259/unable-to-add-multiple-products-to-wishlist), but our issue is not related to pricing or store filters - the records are simply deleted from the database.
Watching the flow, it looks like the Wishlist Item Collection is saved within the Wishlists afterSave() function, and the last item is deleted from the collection.
Not sure why.