theming – Theme a subscriber field in Simplenews issues

I need to address my subscribers in a more personal way. Thus I added fields for Mr./Mrs., firstname, lastname to the subscription block, and data gets collected nicely.
However, I can’t see how to theme these fields in simplenews-newsletter-body.html.twig.

In the header of this template there’s an example printing the subscriber’s (built-in) mail.

 {{ simplenews_subscriber.getMail  }}

Trying the same as {{ simplenews_subscriber.field_lastname }} doesn’t do anything. I tried a lot of variations to this approach, but to no avail.

What’s the secret of getting custom subscriber fields to print with Twig?