When does SQL Server decimal NOT convert to .Net decimal?

-- create a test table in tempdb with one valid and one invalid decimal mapping column.
CREATE TABLE TestTable
( 
    [ID] [INT] IDENTITY(1,1) NOT NULL, 
    [ValidDecVal] [DECIMAL](29, 2) NOT NULL, 
    [InvalidDecVal] [DECIMAL](30, 30) NOT NULL    
)
GO

-- insert some data
INSERT INTO TestTable(ValidDecVal, InvalidDecVal)
-- both values are in correct range
SELECT 123456789012345678901234567.56, 0.123456789012345678901234567890

private void GetData()
{
    using (SqlConnection conn = new SqlConnection(@"server=TestServer; database=tempdb; Integrated Security=SSPI;"))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT ID, ValidDecVal, InvalidDecVal FROM TestTable", conn))
        {
            conn.Open();
            SqlDataReader rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection);
            
            rdr.Read();
            // get the id
            int id = rdr.GetInt32(0);
            // get the 29 precision value just fine
            decimal validDecimal = rdr.GetDecimal(1);
            // this errors out ...
            decimal invalidDecimal = rdr.GetDecimal(2);
            // so does this ...
            decimal invalidDecimal = rdr.GetSqlDecimal(2).Value;

            // the only thing to do is to either pass around SqlDecimal class
            SqlDecimal invalidDecimalAsSqlDecimal = rdr.GetSqlDecimal(2);
            // or to cast it to string
            string invalidDecimalAsString = invalidDecimalAsSqlDecimal.ToString();
            
            // ... do something with upper values
            Console.WriteLine(invalidDecimalAsSqlDecimal);
            Console.WriteLine(invalidDecimalAsString);
        }
    }
}