This is working by design. You need to add view access right to all sub project under the parent project if you would like to open the parent project and its tasks in Open Workbench. Also if the user needs to update tasks under the parent project in Open Workbench, read/write right will be required to the sub project. Please add the same access right as the parent project to the sub project in order to see the parent project in Open Workbench.
In order to open a project in schedulers, Open Workbench and Microsoft Project, 'Project - View' or 'Project - View Management' access right is required as a minimum access right.