Check Usubscribe origin on All Subscribers

In your scenario, (unsub from C, unsub with BU-only context):

• The status in BU C (_Subscribers) will be displayed as "Unsubscribed".
• The status in BU B (_Subscribers) will be displayed as "Active".
• The status in A (ENT._Subscribers) will be displayed as "Active". (You say the contact only exists in B and C - it always exists at the enterprise level.)

Meaning that, if you just look at All Subscribers List, the displayed status is different depending on your perspective, because this was long only accounted for in hidden tables.

You can verify this behavior via the more recently introduced data view "businessUnitUnsubscribes", which will yield NO results in BUs A and B for your user (it only contains subscribers that are "unsubscribed" in your BU), but will show your subscriber in the businessUnitUnsubscribes for BU C.

https://help.salesforce.com/articleView?id=mc_as_data_view_businessunitunsubscribes.htm&type=5

Keep in mind that this behavior is NOT accounted for in MC Connect. "Email Opt Out" is ALWAYS set to true, making it a more or less useless field for your scenario. You can find this confirmed in the documentation: https://help.salesforce.com/articleView?id=mc_co_unsubscribes.htm&type=5

"Marketing Cloud Connect does not support business unit-level unsubscribes for Enterprise 2.0 accounts. "