That can still happen. Just because you can be in many guilds at once does not meet you have to.
Edit: from a guild management standpoint, I think the key to this problem is with the guild management system.
Only allow trusted members to donate plots. Get them on a contract or select people you trust and manage your guild beacons by making sure your trusted members are taken care of. That way, you don’t have a constant flux of beacons being donated/ lost to random players joining or leaving.