Why does BIML script get null value on RootNode.DbConnections?

We are supporting an old system, working on Visual Studio 2015 with BimlExpress 2019 running on Windows Server 2016.

We have the following minimized BIML (business intelligence markup language) script, and it triggers the exception and get error message Source connection not found., which means RootNode.DbConnections["Source"] returned a value of null.

To verify the MySQL database availability, we made a minimized testing C# script to query to the same connection string above for select count(*) from test_source.admin_passwords; and it returns 0 as expected. Therefore, the database looks OK.
 

<Biml xmlns="http://schemas.varigence.com/biml.xsd">
    <Connections>
        <AdoNetConnection Name="Source" Provider="MySql.Data.MySqlClient.MySqlConnection, MySql.Data, Version=8.0.15.0, Culture=neutral, PublicKeyToken=c5687fc88969c44d"
            ConnectionString="server=192.168.56.5;user id=root;port=3306;database=test_source;pwd=mysql_root_password" />
    </Connections>
  
<#
    var sourceConnection = RootNode.DbConnections["Source"];
    if (sourceConnection == null)
    {
        throw new Exception("Source connection not found.");
    }
#>

</Biml>