Setting display to secondary display via preferences has no effect? Bug?

I’m on a Macbook Pro Ventura 13.4.1, hooked up to a secondary display (Dell U2412M) via HDMI, with the Built In Display set as the Main Display in my system settings. Running P4.

I am trying to get sketches to display on my secondary display instead of my built in display by changing the setting in preferences to “Runs sketches on display 2”. I am running the default renderer, with a simple test sketch which only displays a 1200 x 1200 screen. There are other posts talking about issues with secondary displays using fullScreen() or different renderers. But no real resolution (<-- sic).

I have exited and reentered and checked the contents of the Preferences.txt file, which seems to have taken on board the declared setting, as far as declarations go. It contains display.run=2 and run.display=2 - which sounds like it has taken on board the preferences in Settings.

Is there something else I could try, to get this up and running? If this is a bug, how should I raise it? (Not a critical error atm, just a puzzle.)

thanks!