When customers use the Marketing Cloud default Profile Center link to unsubscribe it causes users...

The behavior where customers unsubscribe via the Marketing Cloud default Profile Center and do not receive emails from any other business unit is explained by:

D. Business unit unsubscribe setting causes an Enterprise level unsubscribe. This setting means that when a subscriber opts out using the default Profile Center, they are unsubscribed at the enterprise level, impacting all business units within the organization.