User of Business Unit A is able to share a record with user of other Business Unit B

I created 2 Business Units, Finance and Sales.

I created Event Entity and modified 2 security roles (Finance Manager and Sales Manager) to have Business Unit Level access on Event entity.

I now logged in with finance manager user and created couple of records, and am able to share the records with Sales Manager user who is of another Business Unit (Sales)

Ideally as per configuration on Security roles, should not allow sharing the Event record with other users of other Business Unit.

Please let me why it is behaving this way.

Thanks

KD