What about this solution?

https://mdbootstrap.com/snippets/jquery/mdbootstrap/949080

This is a workaround. I am surprised that such a big library doesn't provide a basic feature. I have dropped the plan to use this library any more.

<div class="btn-group">
    <button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Basic dropdown</button>

    <div class="dropdown-menu">
        <a class="dropdown-item" href="#">Action</a>
        <a class="dropdown-item" href="#">Another action</a>

        <div class="dropdown-submenu">
            <a class="dropdown-item dropdown-toggle" type="button" data-toggle="dropdown" href="#">Something else here</a>

            <div class="dropdown-menu">
                <a class="dropdown-item" href="#">Action</a>
                <a class="dropdown-item" href="#">Another action</a>
                <div class="dropdown-submenu">
                    <a class="dropdown-item dropdown-toggle" type="button" data-toggle="dropdown" href="#">Something else here</a>

                    <div class="dropdown-menu">
                        <a class="dropdown-item" href="#">Action</a>
                        <a class="dropdown-item" href="#">Another action</a>
                        <div class="dropdown-submenu">
                            <a class="dropdown-item dropdown-toggle" type="button" data-toggle="dropdown" href="#">Something else here</a>

                            <div class="dropdown-menu">
                                <a class="dropdown-item" href="#">Action</a>
                                <a class="dropdown-item" href="#">Another action</a>
                                <a class="dropdown-item" href="#">Something else here</a>
                            </div>
                        </div>
                    </div>
                </div>
            </div>
        </div>

    </div>
</div>

.dropdown-submenu {
    position:relative;
}
.dropdown-submenu .dropdown-menu {
    top:0;
    left:100%;
}

$('.dropdown-submenu .dropdown-toggle').on("click", function(e) {
    e.stopPropagation();
    e.preventDefault();
    $(this).next('.dropdown-menu').toggle();
});