flex 父子元素不同direction問題

請問下，flex布局
父元素direction:column,子元素direction:row
子元素內(nèi)的孫(?)元素排列為什么還是column？

    .flex{
        position: absolute;
        height: 200vh;
        top: 0;
        display: flex;
        flex-direction:column;
        align-items:center;
    }
    .flexItemLast{
        flex-direction: row;
        width: 100vw;
        height: 400px;
    }
    .flexItemB{
        width: 30vw;
        height: 80px;
    }
    .flexItem{
        width: 100vw;
        height: 400px;
    }

    <div class="flex">
        <div class="flexItem"></div>
        <div class="flexItem"></div>
        <div class="flexItemLast">
            <div class="flexItemB"></div>
            <div class="flexItemB"></div>
            <div class="flexItemB"></div>
        </div>
    </div>