Having the entire Sass compilation in widgetset module requires that the groovy maven plugin there pulls in magnolia-ui-vaadin-theme. However, it can only pull it in from the local or a remote repository. When building the project, the full reactor build, the newly created theme jar will not have been placed there when the project is built with mvn package or mvn release:prepare. Commonly the plugin does get hold of a theme jar from the local or a remote repository, this is always an older jar from a previous build.

Instead, each module must compile its own scss files.