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?
{{ simplenews_subscriber.field_lastname.value }}
In the header of this template:

Note that depending on the used caching strategy, the generated body
might be used for multiple subscribers. If you created personalized
newsletters and can't use tokens for that, make sure to disable
caching or write a custom caching strategy implemention.

To solve it I added in the sites/default/services.yml
services:
  simplenews.mail_cache: ~
  
  simplenews.mail_cache:
    class: Drupal\simplenews\Mail\MailCacheNone