@Mapper
public abstract class ContactMapper {

    @Mapping(target = "nameInA", source = "nameInB")
    @Mapping(target = "mapped", ignore = true)
    public abstract Contact map(com.example.b.Contact contact);

    @AfterMapping
    void afterMapping(@MappingTarget Contact target, com.example.b.Contact contact) {
        target.setMapped();
    }

    @AfterMapping
    void afterMapping(@MappingTarget Contact.ContactBuilder target, com.example.b.Contact contact) {
        target.mapped( true);
    }
}

import com.example.b.Contact;
import javax.annotation.processing.Generated;

@Generated(
    value = "org.mapstruct.ap.MappingProcessor",
    date = "2024-09-03T09:05:41+0200",
    comments = "version: 1.6.0, compiler: javac, environment: Java 21.0.2 (BellSoft)"
)
public class ContactMapperImpl extends ContactMapper {

    @Override
    public com.example.a.Contact map(Contact contact) {
        if ( contact == null ) {
            return null;
        }

        com.example.a.Contact.ContactBuilder contact1 = com.example.a.Contact.builder();

        contact1.nameInA( contact.getNameInB() );

        afterMapping( contact1, contact );

        // here is the error. contact1.build() will generate a com.example.a.Contact. Due to the import of 
        // com.example.b.Contact the class Contact is from package b.
        Contact contact1Result = contact1.build();

        afterMapping( contact1Result, contact );

        return contact1Result;
    }
}