This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
primaryChildFragment.subtreeFlags =
      currentPrimaryChildFragment.subtreeFlags & StaticMask;
  }
  let fallbackChildFragment;
  if (currentFallbackChildFragment !== null) {
    fallbackChildFragment = createWorkInProgress(
      currentFallbackChildFragment,
      fallbackChildren,
    );
  } else {
    fallbackChildFragment = createFiberFromFragment(
      fallbackChildren,
      mode,
      renderLanes,
      null,
    );
    
    
    fallbackChildFragment.flags |= Placement;
  }
  fallbackChildFragment.return = workInProgress;
  primaryChildFragment.return = workInProgress;
  primaryChildFragment.sibling = fallbackChildFragment;
  workInProgress.child = primaryChildFragment;
  return fallbackChildFragment;
}
function retrySuspenseComponentWithoutHydrating(