(I think I figured this out as I typed out the question... answer below... posting in case it helps someone else).

I have a custom report (for the purpose of detecting imported invoices with over-shipments by 3rd parties) which links invoice OrderLineKey to SO LineKey to SO history LineKey.  Normally this works, but when a backorder is edited to add another line, Sage re-uses previously shipped LineKey values.

The SO history table doesn't use the same primary key as the open SO table, so technically this is not a bug, it's a loophole that breaks the ability to link SO to SO history using the LineKey.

However, the SO_SalesOrderDetail table has a field which I assume is used for this link.