I'm guessing that maybe I forgot to restart the machine to make sure everything got to read it. Or that whatever was broken long ago got fixed.
BTW, I can't reach my backup right now, but this seems like a good start to build up custom mappings in case anyone gets interested in this,
https://github.com/kragen/xcompose
I guess it's up to each Wayland compositor, which calls for inconsistency :/