Default setting is duplex printing at long edge (Portrait). If now choose brochure print the orientation of the page (landscape instead of portrait) is changed. But the duplex printing is not switched from long edge to short edge. I would prefer that the brochure printing change the orientation AND the duplex edge.
If you have set duplex for default it has been set for "duplex long edge", because most of the printing is done for orientation "Portrait". Wen choosing "Page Layout" → "Brochure" the orientation will be changed (right) to "Landscape". Duplex has to be changed at the same time to "duplex short edge", but it won't. The preview doesn't show anything about chosen duplex. If it would be shown you could see every second page reverse with the wrong duplex adjustment. Have tested this with LO 7.2.3.2, also with older version down to LO 6.1.5.2 on the same machine (OpenSUSE 15.2 64bit rpm Linux). Seems it has never been implemented.