Sometimes, the Add All to Cart button appears and then disappears on My Wishlist page. This is not an error but a feature to prevent wrong variant getting into the Shopping Cart.
The button disappears when the App notices that there is at least one item in the Wishlist for which the variant has not been selected.
The Add All button is hidden when both of the following conditions are met:
1. The item has been added to wishlist from collection/category page.
2. The item has more than one variant.
This is done to prevent wrong variants from getting into the Shopping Cart. Otherwise, the “Add All” button is always displayed.