ODF says it with style:page-usage right/left on the section's page layout (read
only where the master is no half of a mirrored pair, which alternates by itself),
Word with a w:type oddPage/evenPage section break; both insert the blank page the
side needs, and so does pagination now — the printed number decides the side.
In the ribbon's Breaks menu under "This section". Measured against the reference
that keeps LibreOffice's own blank pages: the largest fixture 875 → 774 reports,
another 25 → 30 with its page count 27 → 29 against LibreOffice's 32.