List view only shared with partners is visible to internal Users

You've created a list view and shared it solely to all partner Users. However, a few internal Users can still access the list view. 

Under unique scenarios, a User may be the manager of a partner User who is a member of a partner group. In these scenarios, the internal User (Manager) is also considered a member of the partner group.

This membership inclusion marks the internal User (Manager) as being a partner. They aren't actually changed to a partner User, but rather have the partner User indicator for visibility. Therefore, they will be able to view things shared solely to partner Users.