Cannot delete PO Line if other PO Line on the same order is linked to an invoice line

Overview: Originally reported in Iris, reproduced in Iris Bugfest and in Snapshot on 2021-08-12.

Steps to Reproduce:

1. Log in to https://folio-snapshot.dev.folio.org/
2. Go to Settings/Orders/Purchase order lines limit
3. Change the limit from 1 to 2 or more, and Save
4. Go to the Orders app
5. Create a new order
6. Create PO Line 1
7. Create PO Line 2
8. Open the PO
9. In Invoices, create an Invoice and link it to PO Line A
10. In Orders, try to delete PO Line B

Expected Results:
PO Line B is successfully deleted.

Actual Results:
PO Line B fails to delete, with the following errors:
"Error while deleting PO Line"
"This order or order line is linked to Invoice number(s) and can not be deleted"

{
  "errors" : [ {
    "message" : "This order or order line is linked to Invoice number(s) and can not be deleted",
    "code" : "orderRelatesToInvoice",
    "parameters" : [ ]
  } ],
  "total_records" : 1
}

Additional Information:
I get the same error when trying to delete either PO Line, even though, as far as I can tell, only one of them is actually linked to the Invoice line.

URL: https://folio-snapshot.dev.folio.org/

Interested parties: Ben Taylor