Redmine

Hi everyone,
I have a scenario with Issue permission like this:
I have a Project A to store issues (Aa, Ab, Ac) but some issues is sensitive and can only show to some specific users (U1, U2, U3, U4, U5) and cannot show to other members in the project (each issues have their own privacy).

- In issue Aa (related to U1, U2): I set to Private then, change ' Issues visibility ' of U1, U2 to ' All Issue '. Then U1, U2 can see Aa;
- After 3 days, another admin create Issue Ab and Ac with ' Private ' activated and only want to show to U3, U4 so he have to change ' Issues visibility ' of U3, U4 to ' All Issue '.

Now this is a disater : U3, U4 can freely view Issue Aa which I don't want to show them vice versa with Issue Ab, Ac now show to U1, U2. U1, U2, U3, U4 now peeps others Issue and lead to Internal Fraud which I do not want from the beginning.

Hope you guys can understand the scenario above and have a solution.

Thank you, Have a nice day!