Theme a subscriber field in Simplenews issues
{{ 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