Hello Josef,
BSEG gives FI document line items not Invoice wise line items. You can use BSID and BSAD table where you can get all line items. Like example you made payment against 2 invoices. In BSEG ,you can only 2 line items one is Bank line item and other is customer line item. But customer line item consisting of 2 invoices,which same you will get in BSAD table.
Regards
Mani Kumar