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.