Animal Crossing New Horizon, I have a villager on my island who doesn't live here?

I saw Diana a few days ago hanging out in Filbert's yard, I've never played an Animal Crossing game before so idk anything..is this a bug?  Diana was NEVER at my campsite, and I only bought 1 new plot of land for a house and Hopper moved in. Diana does NOT have a house on my Island, so why is she here? Did someone move out without my knowledge and Diana moved in? I'm so confused lol I don't mind cuz she's adorable but idk why she's been on my island if she doesn't have a house here.