alpha-background working in X with picom?

I'm trying to get the new (as of earlier this year) alpha-background frame parameter working, but haven't had luck. I'm using a current build of emacs 29.0.50 on X (with StumpWM/XMonad and picom), it has cairo, built with gtk3, regular alpha works... but no change when setting alpha-background. Can anyone confirm this works for them on X with picom? I've heard it working on Wayland with Sway, and of the emacs fork that had this code initially on X, but what about on the current version of emacs with X? (references: older posts here and https://kristofferbalintona.me/posts/202206071000/ in particular)

Update: the emacs-next package from Guix on my arch machine did not do anything with alpha-background, but the AUR package for emacs-git (same idea, both v29) did work. So likely it is some configuration option in the build that is different. Any thoughts on what is needed or what to look for? Update 2: A build with pgtk (also a Guix package) does have alpha-background doing something, on X. So perhaps a regression on Emacs-side for this property? Or some non-trivial interaction/build option, as I used what I thought were the same configurations options as from the Arch package to test and it didn't work without pgtk.

I have this exact issue with the nixos emacs overlay gcc build.

Hmm! Perhaps suggesting emacs is looking for something in a global location that should be patched to find it in the guix/nix store...