StrSubstitutor.replaceSystemProperties does not work consistently

package test.utilities;

import java.util.Properties;

import org.apache.commons.lang.text.StrSubstitutor;

public class TestStrSubstitutor {

    public static void main(String[] args) {
        System.out.println(StrSubstitutor.replaceSystemProperties("os.name=${os.name}"));
        Properties testProps = new Properties();
        testProps.put("test_key",  "test_value");
        testProps.putAll(System.getProperties());
        System.setProperties(testProps);
        System.out.println(StrSubstitutor.replace("test_key=${test_key}", System.getProperties()));
        System.out.println(StrSubstitutor.replaceSystemProperties("test_key=${test_key}"));
    }

}

os.name=Windows 7
test_key=test_value
test_key=${test_key}