I think there is a problem with rights management.
If USER-A a member of GROUP-1 then he can only edit his absences - okay
If USER-A a manager of GROUP-2 then he can edit the absences of all the members of GROUP-2 - okay
If USER-A a member of GROUP-1 and manager of GROUP-2 then he can edit the absences of all the members of GROUP-2 and of GROUP-1. But he is only member of GROUP-1 and should the absences GROUP-1 can not edit.