In short, go crazy with plot removal tool, there is some strange issue like settlement is taking plots but they do not belong to anyone. You can remove them with plot removal tool, but it’s kinda tricky since those plots can be underground and connected with other non-obvious “settlement plots”. Once you remove those plots, they actually do not belong to any beacon, you’ll be able to expand your beacon.
One question, how far is that plot from original location where you setup campfire?