.mobile-menu-new{

}
.mobile-menu-new__ul{

}
.mobile-menu-new__ul>li{
    border-bottom: 1px solid rgba(0, 0, 0, 0.06);
    display: block;
    padding: 16px 0;
    font-size: 18px;
    color: #2d2d2d;
    text-decoration: none;
    border-bottom: 1px solid rgba(0, 0, 0, 0.06);
    transition: color 0.2s;
    position: relative;
}
.mobile-menu-new__ul li a{
    font-size: 18px;
}

.mobile-menu-new__ul li ul{
    display: none;
}

.open-sub-menu {
    display: block;
    width: 20px;
    height: 20px;
    position: absolute;
    right: 0;
    top: 20px;
    transform: rotate(180deg);
    transition: transform 0.5s ease; /* Вот это всё решает! */
}

.open-sub-menu.active {
    transform: rotate(0);
}