I'm using the config file provider for Maven settings.  The resulting settings file should have a placeholder that will be replaced by Maven.  However, the Maven format this ( ${env.HOME}) causes it to get replaced by some interaction between this plugin and the configuration as code.

If the CASC source file has 

<username>${env.USERNAME}</username>

The config file provider has that line as

<username></username>

This is the content of the file when viewed in Manage Jenkins > Managed files > [configuration file].  So this must be happening during the process of applying a new configuration.   I've confirmed that the text is missing in the plugin's config file (org.jenkinsci.plugins.configfiles.GlobalConfigFiles.xml).

This appears to be a change of behavior with the new version of the configuration file provider plugin.  It was working for me with a previous (probably 2.18) version.