Replace Connection String with Web.Release.config using external configSource

Question

I'm trying to move all of our connection strings to external files but I can't get our production app to run - when publishing we receive an Error that says:

**

Web.config Connection String argument cannot be null or empty.

**

I wonder if the Release transform is not working.

FYI - we have all file properties set Copy To Output Directory to Copy Always

We have 5 files -

1) Web.config
2) Web.Release.config
3) Web.Debug.config
4) Configurations\Debug.Conn.config
5) Configurations\Release.Conn.config

For Development: (this is working)

Web.config & Web.Debug.config contains:

<connectionStrings configSource="Configurations\Debug.Conn.config" />

The config source is pointed to our local dev database and is working:

Configurations\Debug.Conn.config:

<connectionStrings>
  <add name="AppDB" 
    connectionString="Data Source=DESKTOP-ABCDEF;Initial Catalog=MyApp;Integrated Security=True" 
    providerName="System.Data.SqlClient" />
</connectionStrings>

For Production: (This is not working)

Web.Release.config contains:

<connectionStrings xdt:Transform="Replace" configSource="Configurations\Release.Conn.config" />

The configSource is pointed to our Production Database.

Configurations\Release.Conn.config:

<connectionStrings>
  <add name="AppDB" connectionString="Data Source= ... Production Database ... '" />
</connectionStrings>

--

Firstly: In all my searching for an answer I was only able to find posts regarding transformations using the Web.config main files - barely any mentioned using external files using configSource

Secondly: The transformations work when I remove the external files and put the data directly into the Web.config files - but that defeats the purpose of what we're trying to do.

So - I can't tell what is missing to get the Production DB from replacing the connectionStrings when publishing.