Custom gradle sourceset no longer recognized as source root folder

Hi, 

We have a project with an extra custom gradle SourceSet defined: 

After import the project into Intellij Idea (without "create separate modules per sourceset"), the generator sourceset's java folder is no longer recognized by Intellij as source root. Build with delegating to gradle is fine, but IDE build fails, and those java files are treated as text files when opened in editor. We have this setup for about two years now and this recently becomes broken. I don't know which version it started though. Anyone seeing the same? 

sourceSets {
    generator {
        java {
            srcDir 'src/generator/java'
        }
    }

    main {
        java {
            srcDir "build/generated/main/app/java"
        }
    }

    test {
        java {
            srcDir "$build/generated/test/app/java"
        }
    }
}