When using compile-time enhancement, PluralAttribute.getPersistentAttributeType() returns PersistentAttributeType.ONE_TO_MANY for all plural (collection) associations.

The problem, maybe, is inside:

// line 929
public static boolean isManyToMany(Member member) {
	if ( Field.class.isInstance( member ) ) {
		return ( (Field) member ).getAnnotation( ManyToMany.class ) != null;
	}
	else if ( Method.class.isInstance( member ) ) {
		return ( (Method) member ).getAnnotation( ManyToMany.class ) != null;
	}

	return false;
}

where Field.getAnnotation(ManyToMany.class) is called on the enhanced/generated field which doesn't have the annotation.

It should be called on the original field.