Not sure if this can even be fixed but I'm hoping it can since I just spend the last hour trying to create a custom shield only to have the graphics fail when I tested.
Item Editor -> Shields: Link Tile Offset doesn't recognize a value of 1040 which is the next page after the Mirror Shield. Tested this both using the needed value of 1040 (for the shield I made) and changed it back to 780 (mirror shield) Mirror shield graphic worked fine, but it would not recognize the new sprites set up for the higher value. I basically copy-pasted all the sprites from the Z3 Magic Shield section then modified the tiles that show the shield, taking great pains to make sure the tiles were exactly where they were in relation to the page I took them from, and all of them were carried over. The result was a shield-less link during testing with the larger value.