[2020] Each buckler from The Everlight mystery is dropped on a different island btw

This map is correct. For an area, it includes all spots including material caches. The mass grave island's unique drop is the icyene remains. The underwater cave gives mosaic pieces. You don't need to believe this, but if you are searching for a specific buckler, at least try following this map first. If anyone has evidence this is not the case, please do tweet me, but memory is an unreliable form of evidence.