Assignment A Activity completion is set to: 'Student must submit to this activity to complete it'
Assignment B Restrict access is set to: 'Activity completion = Assignment A must be completed with passing grade'.
Result: Student cannot access Assignment B eventhough he/she submitted Assignment A and got passing grade. To fix this, one should edit Assignment A completion rule and enable 'Student must receive a grade to complete this activity'. So it seems the grade is not exposed to access restrictions checks when it's not ticked in respective completion.
Although this is probably logical, but the system should at least warn you when setting such a restriction on Assignment B, or this option should be inaccessible.