I have no idea why this whole tent thing is causing such issues. But my take on it is that maybe when you first move into the town there is not a house for you to live in yet, and you start out in a tent only until maybe you do something for that real estate tycoon now know as "Nook" such as buy a plot of land to put the tent on, and then after you do a few things you can upgrade to a house? Why else would it have a mailbox by it.
However, I do hope that it means that since your tent is in the game, I honestly hope the igloos and summer tents return, cause I loved playing those games from the GCN version. And correct me if I am wrong, but the more I learn about the game, the more it seems like a GCN remake on steroids.