Config overrides only work with hard-coded values, cannot use environment variables

Problem/Motivation

We typically use environment variables in our local, development, staging and production environments to store credentials for various drupal modules and services, and configure overrides in settings.php to utilize the values stored in environment variables as config override values for different environments.

Every time we configure an environment variable to store anything other than the 'smtp_host', sending mail from the site fails.

Hard-coding those configuration override values in settings.php, however, works.

To make this work unilaterally, requires that we add extensive logic for each hard-coded value that changes per environment, making a standard configuration workflow much more friable.

Steps to reproduce

Configure a config override in settings.php for the 'smtp_port' with a hard-coded value, and reset the cache.
Test sending an email and verify that this is successful.

Set an environment variable called "SMTP_PORT" to the same value as the override value above.
Change the config override in settings.php to read the environment variable instead, using getenv('SMTP_PORT'), and reset the cache.
Test sending an email -- this will fail.

Scratching my head on this one.