Updating account on opportunity causes owner sharing record get deleted

Workflow updates account owner to generic user 'SFDC Admin' when ever account are created by users. with that said.. If any user is updating account on opportunity, it is resulting in deletion of sharing record with reason -'Owner' and opportunity access -'Full Access'. After this only one sharing record persists and i.e sharing record with reason 'Account Sharing' and Opportunity access 'Read/Write', this is stopping opportunity owner to edit his own opportunity. will the opportunity sharing records get deleted when account is updated? if so what would be way to solve this problem.