Interestingly, with call hero and dominate you can end up owning all of them fairly early on (Call Hero repositions heroes currently on the map).

Don't know if it's a global issue but the two closest monster caves to the Frostling starting location have the status 'explored'.

If you editor-open the map from heavengames, it gives an error message stating something along the lines of "missing asset 64. Ingore?." I suspect he used his mod to make his map and that his mod has changes to the default item library - instead of adding the custom items to the map itself. If the map was told "look in the default list for item 64'" and there was no item 64 in the unmodded list, I think the game just doesn't render an item.

Or maybe it's an issue of too many caves with not enough items available. If you open the map from heavengames and check that cave to see if it's set to random or custom items, it will give a better indication of what happened. Interesting find