Multimethod groovy classes behave differently when called from java compared to Groovy

I have a mixed groovy/java application where the domain model is implemented in groovy, and the java utilizes the groovy classes.
I'm trying to use the multimethods functionality of groovy to do some nifty stuff, such as writing a simpler equals method.
I have two scenarios of usage;

• The groovy and java classes are in the same module
• The application is multimodule, where code from a module depends on the groovy code, from a jar

My problem is that the calling java code does not behave in the same way groovy calling code would. For example, different equals methods are apparently used.
Why is this? Example attached.
I guess this could be normal Groovy behavour, but is in which case something to beware of with mixed projects

Is there any special way to package the groovy code (not as compiled class files), such as uncompiled .groovy files, that makes multimodule projects behave as wished?