【PowerMock】System.getenv().get で null が返る

Question

PowerMock を使って System.getenv().get をモック化しようとしています。

テスト対象のクラス:

public class SSOClient {

  private SSOClient() {};

  public static Keycloak getInstance(){

      return Keycloak.getInstance(System.getenv().get(SSOConstants.KEY_SERVER_URL),
              System.getenv().get(SSOConstants.KEY_REALM),
              System.getenv().get(SSOConstants.KEY_USERNAME),
              System.getenv().get(SSOConstants.KEY_PASSWORD),
              System.getenv().get(SSOConstants.KEY_CLIENT_ID),
              System.getenv().get(SSOConstants.KEY_CLIENT_SECRET));
  }
}

テストクラス:

@RunWith(PowerMockRunner.class)
@PrepareForTest({Keycloak.class})
public class SSOClientTest {

  private static String SERVER_URL = "localhost";
  private static String REALM = "realm";
  private static String USERNAME = "username";
  private static String PASSWORD = "password";
  private static String CLIENT_ID = "client_id";
  private static String CLIENT_SECRET = "client_secret";

  @Before
  public void setUp() throws Exception {

    PowerMockito.mockStatic(Keycloak.class);

    Map<String, String> envMap = new HashMap<>();
    envMap.put(SSOConstants.KEY_SERVER_URL, SERVER_URL);
    envMap.put(SSOConstants.KEY_REALM, REALM);
    envMap.put(SSOConstants.KEY_USERNAME, USERNAME);
    envMap.put(SSOConstants.KEY_PASSWORD, PASSWORD);
    envMap.put(SSOConstants.KEY_CLIENT_ID, CLIENT_ID);
    envMap.put(SSOConstants.KEY_CLIENT_SECRET, CLIENT_SECRET);
    PowerMockito.mockStatic(System.class);
    PowerMockito.when(System.getenv()).thenReturn(envMap);
  }

  @Test
  public void testGetInstance() throws Exception {

    Keycloak client = SSOClient.getInstance();

    PowerMockito.verifyStatic(Keycloak.class, VerificationModeFactory.times(1));
    ArgumentCaptor<String> serverUrl = ArgumentCaptor.forClass(String.class);
    ArgumentCaptor<String> realm = ArgumentCaptor.forClass(String.class);
    ArgumentCaptor<String> username = ArgumentCaptor.forClass(String.class);
    ArgumentCaptor<String> password = ArgumentCaptor.forClass(String.class);
    ArgumentCaptor<String> clientId = ArgumentCaptor.forClass(String.class);
    ArgumentCaptor<String> clientSecret = ArgumentCaptor.forClass(String.class);

    Keycloak.getInstance(serverUrl.capture(), realm.capture(), username.capture(), password.capture(), clientId.capture(), clientSecret.capture());

    // これは通ります
    assertThat(System.getenv().get(SSOConstants.KEY_SERVER_URL), is(SERVER_URL));

    assertThat(serverUrl.getAllValues().get(0), is(SERVER_URL));
    assertThat(realm.getAllValues().get(0), is(REALM));
    assertThat(username.getAllValues().get(0), is(USERNAME));
    assertThat(password.getAllValues().get(0), is(PASSWORD));
    assertThat(clientId.getAllValues().get(0), is(CLIENT_ID));
    assertThat(clientSecret.getAllValues().get(0), is(CLIENT_SECRET));
  }
}

assertThat(System.getenv().get(SSOConstants.KEY_SERVER_URL), is(SERVER_URL)); は通るにも関わらず、それ以降の System.getenv().get で Null が返ります。

java.lang.AssertionError: 
Expected: is "localhost"
     but: was null
```

何か使い方を間違っていますでしょうか？
ご教示いただけますと幸いです。