I noticed this problem while trying to print something today. Even though the page was formatted to landscape, the printer would still print it in portrait mode.
My printer is a Brother HL-2170W using the Foomatic/hpijs-pcl5w (recommended) driver.
In LibreOffice, go to File > Printer Settings > (select printer) > Properties > Device.
Change ‘Printer Language Type‘ from ‘PDF’ to ‘PostScript (Level from driver)’
The page now prints in the correct orientation.