EnvVarContextResolver returns full expression string instead of fallback value

[kfh]

I'm using HOCON as the configuration file format.

A short extract:

kafka {
  bootstrapServers = "${{ env:BOOTSTRAPSERVERS:-fallback }}"
}

When the BOOTSTRAPSERVERS env var is defined the correct value is returned. When it's missing the whole expression string is returned instead of the fallback value: ${{ env:BOOTSTRAPSERVERS:-fallback }}.

This was not an issue with the EnvOrSystemPropertyPreprocessor.