Installing transmission-cli from the world repository (current version 4.0.1-2) does not include any web interface files. This has been noted by others here on reddit (https://old.reddit.com/r/artixlinux/comments/11ip6od/transmission_webui_borked/).
A usr/share/transmission/ folder is not created upon installation with the necessary web interface files.
Update: solved in current version (4.0.2-1).