I often get that error when I export my various maps. However, when I give out the maps to other people it doesn't seem to be an issue. They see everything that I see, and I have checked. I don't believe there's an issue. My guess is if you went out of your way to get the items, the creator might have left something out that wasn't extremely vital. Or it's just a feralheart glitch!
When you export your maps and give them out for others to use, do they see all of the objects that you see? If they do, it shouldn't be an issue! x3!