This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
if (current === null) {
    
    
    
    if (nextProps.fallback !== undefined) {
      tryToClaimNextHydratableInstance(workInProgress);
      
      if (enableSuspenseServerRenderer) {
        const suspenseState: null | SuspenseState =
          workInProgress.memoizedState;
        if (suspenseState !== null) {
          const dehydrated = suspenseState.dehydrated;
          if (dehydrated !== null) {
            return mountDehydratedSuspenseComponent(
              workInProgress,
              dehydrated,
              renderLanes,
            );