Seeking clarity on nested object behavior in 2.5

If MyEntity has a nested class, it throws org.bson.codecs.configuration.CodecConfigurationException: Can't find a codec for CodecCacheKey{clazz=class com.foo.Address, types=null}. if the nested class is not annotated with @Entity.

This is different from 1.3 and results in a breaking change when upgrading to 2.5.

Is this a bug or intentional behavior? If intentional, is there a workaround to follow 1.3?