AHAHAH. I know what this is, it's glorious!
When you try to link your maps to an official FH one, you'll be linked to a version of that map with no people. Kind of like your own private version of that map.
I had this happen when I started making maps, I wanted an exit portal to Fluorite Plains... And so I put one in. Me and my friends were the ONLY people in our own private Fluorite Plains. Anyone that goes through your portal will be there with you.
It can also happen as a glitch, in some cases, like yours. It's rare, but you had the Cape to yourself - you lucky dog/cat. Or... there could have just been no people on. *shrugs*