Duplex printing on other than A4 not working

LO Version: Build ID: 1:6.1.5-3+deb10u7 on MX Linux
Printer: ENVY 5640 series wireless connected

I have a multi page document that I want to print as a brochure but smaller than A5 on A4 paper - ie so I trim the excess paper to create the size I want.
It seems that the Duplex function, although turned on and set to bind on the short edge, doesn’t function unless I use A4 as the paper size, which produces an A5 brochure which I don’t want!
Is there any solution to this?

When I need to produce non-standard sized documents, I cheat: I use standard size and I adjust the margins to match my target format. Caution! With duplex printing, the margins are different on odd and even pages. Take this into account when defining your page style(s).

You can “trim the excess paper to create the size” you want, then print duplex.