I have an account with related contacts. When I share/unshare an account all related contact records are also shared/unshared. If I unshare a contact record and login to the user with revoked access to this record, this user still has access to it.
This is probably due to a cascading Entity relationship behavior.
When a one-to-many entity relationship exists there are cascading behaviors that can be configured to preserve data integrity and automate business processes.
These configuration options are called cascading behaviors because they cascade down the hierarchy of related entities. For example, if deleting an account causes related opportunities to be deleted, what about the activities associated with the opportunities? In Microsoft Dynamics 365 the behavior defined in each of the entity relationships for activity type entities is that they are deleted as well.
Check what settings you have for Share on the relationships between account and contact. It's probably set to Cascade.
Action - Share
Description - When the referenced entity record is shared with another user.
Valid Options (Values) - Active, Cascade, NoCascade, UserOwned.
Value | Description
Active | Perform the action on all active referencing entity records associated with the referenced entity record.
Cascade | Perform the action on all referencing entity records associated with the referenced entity record.
NoCascade | Do nothing.
UserOwned | Perform the action on all referencing entity records owned by the same user as the referenced entity record.