I've tested this on XFCE (X11) with two screens - I can confirm this fixes the issue.
I've also tested that this works correctly on:
Wayland: KDE Plasma, GNOME, XFCE (experimental), Cinnamon (experimental)
X11: LXQt, Mate, Cinnamon

(This however may just be a GTK bug too).

Maybe blaming this on GTK was too hasty - it only seems to happen on XFCE.

@deltragon It works! Tested in Linux Mint 22.2 XFCE.

I wonder, how did you even figure out that switching the order of the two lines would fix the issue?

@archisman-panigrahi Well, as I was speculating that this might be a bug in GTK, I was reading the source code of GTK.
And there I found that in gtk_window_fullscreen_on_monitor, there's different code that runs whether the window is "mapped" (i.e. shown) or not. Then I just figured I'd try what happens.