With the update from gtk3-3.24.24-1.fc33 to gtk3-3.24.25-1.fc33 GTK applications stopped loading my .XCompose file.
gtk3-3.24.25-2.fc33 solves it. https://bodhi.fedoraproject.org/updates/FEDORA-2021-51ebafece6
FEDORA-2021-51ebafece6 has been submitted as an update to Fedora 33. https://bodhi.fedoraproject.org/updates/FEDORA-2021-51ebafece6
FEDORA-2021-51ebafece6 has been pushed to the Fedora 33 stable repository. If problem still persists, please make note of it in this bug report.