Still 2024 and select multiple is still not working. I am using Blazor and .Net core 8. I had to make one from scratch and I share it for anyone who has the same problem. This is a very basic design, so you can modify to suit your needs.

<div class="dropdown">
    <button class="dropdown-toggle mr-4" data-toggle="dropdown" type="button" @onclick="() => show = !show "
    aria-haspopup="true" aria-expanded="false">
        @Tip
    </button>
    @if (ListLabels != null){
        <div class="dropdown-menu @(show? "show":"")">
            <div class="form-group row">
                @for (int i = 0; i < ListLabels.Length; i++){
                    <div class="@GetCol(NumCols)">
                        <label class="form-check-label">
                            <input class="form-check-input"
                            type="checkbox"
                            checked="@list_values[i]"
                            @bind-value="list_values[i]">
                            @ListLabels[i]
                        </label>
                    </div>
                }
            </div>
        </div>
    }
</div>

@code 
{
    [Parameter]
    public string Tip { get; set; } = string.Empty;

    [Parameter]
    public int NumCols { get; set; } = 1;

    [Parameter]
    public string[] ListLabels { get; set; }

    bool[] list_values = null;
    [Parameter]
    public bool[] ListValues
    {
        get => list_values;
        set
        {
            list_values = value;
        }
    }

    bool show = false;

    string GetCol(int value)
    {
        switch (value){
            case 1: return "col-12";
            case 2: return "col-6";
            case 3: return "col-4";
            case 4: return "col-3";
            case 6: return "col-2";
            case 12: return "col-1";
            default: return "col-12";
        }
    }

    protected override void OnParametersSet()
    {
        if (ListLabels != null){
            if (list_values == null || list_values.Length != ListLabels.Length){
                list_values = new bool[ListLabels.Length];
            }
        }
    }
}

To use it is very simple

<InputMultiSelector ListLabels="list_labels" ListValues="list_checks" Tip="Choose your pizza" NumCols="3"/>
@code 
{
    bool[] list_checks = new bool[6] { false, true, true, false, false, true };
    string[] list_labels = new string[6] { "cheese", "tomatoes", "mozarella", "mushrooms", "pepperoni", "onions" };
}