So the only way I caught this was because I noticed things on my plot have vanished from the furnaces and that my doors that I left shut were open. Long story short: My bf was messing on my plot and found out he could get into my furnaces without being added to the plot. Not sure if this should even be a thing? Or if its a glitch? If this is a glitch I can make a report ticket or whatever on it.
You should be able to do /arm gui and change the settings of your region to your specifications. Alternatively, you can put shop signs on your furnaces so that no one can access them other than yourself.