@TestParameter fields are only injected when the test class is run with @RunWith(TestParameterInjector.class). If the test class uses another runner such as @RunWith(JUnit4.class), JUnit does not initialize @TestParameter fields, leaving them at their default Java values (null, 0, or false).

@RunWith(JUnit4.class)
public final class MyTest {
  @TestParameter boolean enabled;

  @Test
  public void myTest() {
    // `enabled` is always `false`!
  }
}

@RunWith(TestParameterInjector.class)
public final class MyTest {
  @TestParameter boolean enabled;

  @Test
  public void myTest() {
    // Runs with `enabled = true` and `enabled = false`.
  }
}