<style>
    /*should work but does not*/
    .high-zindex {
        z-index: 124;/*note how this is higher than the z-index of the div element, and is higher than the default z-index of the component*/
    }

    /*workaround*/
    .k-animation-container {
        z-index: 15000;
    }
</style>

<div style="position: absolute; z-index: 123; width: 600px; height: 200px; background: yellow;">
    <TelerikMultiSelect Data="@Countries"
                        @bind-Value="@Values"
                        Placeholder="Enter Balkan country, e.g., Bulgaria"
                        ClearButton="true" AutoClose="false"
                        PopupClass="high-zindex">
    </TelerikMultiSelect>
</div>

@code {
    List<string> Countries { get; set; } = new List<string>();
    List<string> Values { get; set; } = new List<string>();

    protected override void OnInitialized()
    {
        Countries.Add("Albania");
        Countries.Add("Bosnia & Herzegovina");
        Countries.Add("Bulgaria");
        Countries.Add("Croatia");
        Countries.Add("Kosovo");
        Countries.Add("North Macedonia");
        Countries.Add("Montenegro");
        Countries.Add("Serbia");
        Countries.Add("Slovenia");

        base.OnInitialized();
    }
}