Setting "auth_cache_verify_password_with_worker = yes" in order to leverage multiple cores for the Dovecot authentication-process causes Dovecot not to proxy anymore. With debug-logging I figured:

auth_cache_verify_password_with_worker = no

passdb out: OK 1 user=username host=bla port=10993 ssl=any-cert mail_crypt_global_public_key=key mail_crypt_global_private_key=otherkey  hostip=123.123.123.123 proxy pass=pw

auth_cache_verify_password_with_worker = yes

passdb out: OK 1 user=username

The rest seems missing.

Dovecot version: v2.3.11.3

Searching for this issue, I found at least two posts mentioning the same symptoms:

https://dovecot.org/pipermail/dovecot/2018-April/111583.html
https://dovecot.org/pipermail/dovecot/2020-April/118564.html
https://listen.jpberlin.de/pipermail/dovecot/2020-April/001915.html (same as the english one from april 2020)​​​​​