I imagine it should have been seqPos-(idPos+2)

seems to be like this since the initial commit of

commit 091fe76b
Date:   Tue Jun 30 12:55:18 2015 +0300

    tdf#87348 implement mso-next-textbox vml-style textbox chaining import

Change-Id: Ic2f527ede2102c01c8589d58d8c705d59b0a6ffe
Reviewed-on: https://gerrit.libreoffice.org/67453
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>