Invoice line level approval with multiple requesters

We have PO matched invoices with multiple lines and multiple requesters. We setup AME invoice line level as well as header level rules so workflow sends notifications to line level requesters for approval, then to header requester for approval.

For example, we have a PO matched invoice with two lines, header requester A,  line 1 requester B and line 2 requester C. After starting the workflow, the "View Invoice Approval History" stated that invoice line approval: B, Sent; invoice line approval: C, Future; invoice approval: A, Future which were all correct. However, after B approved it through the notification, system skipped C and sent a notification to A for approval. Invoice line 2 with C as the requester has Approval Not required. Why is that?