This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
current: Fiber,
  workInProgress: Fiber,
  renderLanes: Lanes,
) {
  
  reconcileChildFibers(workInProgress, current.child, null, renderLanes);
  
  const nextProps = workInProgress.pendingProps;
  const primaryChildren = nextProps.children;
  const primaryChildFragment = mountSuspensePrimaryChildren(
    workInProgress,
    primaryChildren,
    renderLanes,
  );
  
  
  primaryChildFragment.flags |= Placement;
  workInProgress.memoizedState = null;
  return primaryChildFragment;
}
function mountSuspenseFallbackAfterRetryWithoutHydrating(