Custom User Role cannot edit the Parent Post until the assigned Child Post is controlled by Access as well.

Symptoms

Currently, if post types are involved in a post relationship, Toolset Access needs to control all the related post types in order to control the capabilities of Users over those post types.

If you control only one end of the relation, the users capabilities that are set in Access will not apply.

Workaround

When working with post relationships make sure to always put all the related post types under Access control.