When I try to share a folder using the share tab in file properties, if I go back and remove sharing options from the Share tab, it also affects the Security tab, removing inheritance of permissions from the parent and also removing the Authenticated Users permission, causing the little lock icon on the folder to appear.
So I think I partially understand it, but at the same time, not really. I get that sharing a folder changes sharing permissions, but I'm not entirely sure why it is changing local permissions. I'm not smart enough to figure this out on my own, sorry.