This article explains how backup-person inheritance works.
User C is backup person for user B, who simultaneously is backup person for user A. User A has received an invoice that is routed to User C who can not approve the invoice due lack of user rights.
This works as designed. The approval rights inherit only once.
Example: User A has user B as backup, and B has user C as backup. invoice comes to user A, it gets routed to user B (with A's rights), and then it gets routed to C (without rights from user A). This is to prevent for example CEO's rights to accidentally inherit to summer trainee