3.0 documentation says that private screens are visible to their owner and the users the screen is shared with. I guess that user should not see these private screens, if he doesn't meet the conditions above.
Here comes the problem: if regular Zabbix Admin (non-Super), who is neither the screen owner nor the one the screen is shared with, has at least read permissions to all screen elements, he can see the screen, edit it and even change screen's owner.
Please close this issue, if it is an expected behaviour.