After removing the workaround from my system configuration file and reconfiguring the system with a recent Guix (see below), things work find in MATE, Xfce and sway. I can't reproduce the problems anymore.
$ guix describe Generation 72 Apr 17 2020 08:12:14 (current) sirgazil-x 66d4b67 repository URL: https://gitlab.com/sirgazil/guix-channel-x.git branch: master commit: 66d4b677875c84d0b7a946376cd4885f202094eb guix 79ef072 repository URL: https://git.savannah.gnu.org/git/guix.git branch: master commit: 79ef0726a9afb7bd4676b9a53401f742c7b83653 Thanks for fixing this.