iPhone,HTML上的菜单消失

If you check out https://slikkness.com/home. and you open the menu and scroll down the menu will disappear.

HTML:

        <a class="navbar-brand  mr-auto font-weight-bold" href="#home">SlikkneSs
        </a>

        <a href="#" class="navbar-toggler d-block  ml-auto" data-toggle="collapse" aria-expanded="false" aria-label="Toggle navigation">
            <span class="menu-text">Menu</span>
            <span class="close-text">Close</span>
            <span class="open-text">Open</span>


            <svg class="ham hamRotate ham1" viewBox="0 0 100 100" width="55">
                <path class="line top" d="m 30,33 h 40 c 0,0 9.044436,-0.654587 9.044436,-8.508902 0,-7.854315 -8.024349,-11.958003 -14.89975,-10.85914 -6.875401,1.098863 -13.637059,4.171617 -13.637059,16.368042 v 40"></path>
                <path class="line middle" d="m 30,50 h 40"></path>
                <path class="line bottom" d="m 30,67 h 40 c 12.796276,0 15.357889,-11.717785 15.357889,-26.851538 0,-15.133752 -4.786586,-27.274118 -16.667516,-27.274118 -11.88093,0 -18.499247,6.994427 -18.435284,17.125656 l 0.252538,40"></path>
            </svg>
        </a>

        <div class="menu-list">
            <div class="menu-list--content">
                <div class="menu-list--left">
                    <ul>
                        <li><a class="text-inverse nav-link active" href="/">Fortnite Shop</a></li>
                        <li><a class="text-inverse nav-link" href="/giftcards">Gift Cards</a></li>
                        <li><a class="text-inverse nav-link" href="/games">Games</a></li>
                    </ul>
                </div>

                <div class="menu-list--right">
                    <div class="menu-list--social">
                        <h3 class="mb-5">Get in Touch</h3>

                        <ul class="text-left my-4 list-unstyled list-inline">
                            <li class="list-inline-item"><a class="hover-state" href="https://twitter.com/slikkness"><i class="fab fa-twitter"></i></a></li>
                            <li class="list-inline-item"><a class="hover-state" href="https://instagram.com/slikkness"><i class="fab fa-instagram"></i></a></li>
                        </ul>
                    </div>
                </div>
            </div>
        </div>
    </div>
</nav>

我认为它仅适用于iPhone,我认为一切都很好吗? 在PC和Mac上,一切都很好,但是对于移动设备(我认为是iPhone),它就坏了。