Settings.metaClass.mixin doesn't work as expected

Hi, I use groovy mixin functionality in a project. Since gradle version 3.4 it doesn’t work. I can’t use some predefined properties inside mixin such as ‘project’, ‘gradle’ and so on. It doesn’t work on Linux and macOS but works on Windows. It also works if I replace the property call with get… method: ‘gradle.ext.buildLibPath’ to ‘getGradle().ext.buildLibPath’ for example. I created simple project for demonstration:
Is it a bug or I do something wrong?