Hello!
The party permissions affect players in your party, your parties ally's and enemies (so other parties), and “neutral” is anyone who isn’t in your party, but isn’t an ally or an enemy.
It’s these perms you’ll want to be careful of when you’re building on your claim. If these were open then yes, that may be why.
If you DM me the coords of where you were griefed/stolen from, I can send it to the Survival mods to see what they can do in terms of helping to get your items back.