If the parent element writing-mode = vertical-lr, then the writing-mode values in the child elements, initial and horizontal-tb, show different styles
.pop_menu {
display: flex;
flex-flow: row wrap;
justify-content: space-between;
writing-mode: vertical-lr;
height: 100%;
.pop_menu_item {
display: inline-block;
flex-basis: 35%;
margin-bottom: 20px;
writing-mode: initial;
}
}