https://docs.gradle.org/current/release-notes.html#location-of-classes-in-the-build-directory documents a breaking change :

The default location of classes when using the java .. plugin has changed from:
Java, `src/main/java` -> build/classes/main
..
to
..
Java, `src/main/java` -> build/classes/java/main

This breaks the FX build.
This change is to support multiple language output locations.
Since we don't need that, the most prudent/safest thing is to
change the output location back to what gradle 3.X used.
This will be compatible with 3.x and so we can use either gradle 3.x
or gradle 4.x