outbound mail smtp password gets corrupted on wordpress
I might have found the answer while browsing https://docs.civicrm.org/sysadmin/en/latest/requirements/#php-extensions.


  PHP Mcrypt [is] required for SMTP credentials and other credentials in the database. If you add Mcrypt on after running CiviCRM for a while you will need to re-save your passwords to the database. [... php7.2] does not support mcrypt and if mcrypt is not installed the smtp password (if entered) will need to be re-saved once you update your php version to 7.2.


I'm reasonably sure that this happened to me multiple times on the same time (that has only been upgraded to php7.2 once) but it certainly feels like a pointer in the right direction.