 
	
The context DataItem is null for subsequent ChartSeries. It receives the correct object for the first ChartSeries, but for the rest is null.
<Admin Edit>
A workaround, for the time being, would be to remove the Template and let the Tooltip display the default value, which will be correct.
</Admin Edit>
Reproduction code:
<TelerikChart>
    <ChartTitle Text="Unrecoverable Errors Per Minute vs. Signal Level" />
    <ChartCategoryAxes>
        <ChartCategoryAxis Type="@ChartCategoryAxisType.Category" />
    </ChartCategoryAxes>
    <ChartValueAxes>
        <ChartValueAxis>
            <ChartValueAxisLabels Visible="true" />
        </ChartValueAxis>
    </ChartValueAxes>
    <ChartTooltip Visible="true">
        <Template>
            @{
                var data = context.DataItem as ModelData;
                <div class="card" style="font-size: 0.8rem; color: black">
                    <span>@($"{data.Value}")</span>
                    <span>@($"{data.Type}")</span>
                </div>
            }
        </Template>
    </ChartTooltip>
    <ChartSeriesItems>
        <ChartSeries Type="ChartSeriesType.Line"
                     Data="@ChartData1"
                     CategoryField="@nameof(ModelData.Type)"
                     Field="@nameof(ModelData.Value)">
        </ChartSeries>
        <ChartSeries Type="ChartSeriesType.Line"
                     Data="@ChartData2"
                     CategoryField="@nameof(ModelData.Type)"
                     Field="@nameof(ModelData.Value)">
        </ChartSeries>
        <ChartSeries Type="ChartSeriesType.Line"
                     Data="@ChartData3"
                     CategoryField="@nameof(ModelData.Type)"
                     Field="@nameof(ModelData.Value)">
        </ChartSeries>
    </ChartSeriesItems>
</TelerikChart>
@code {
    public class ModelData
    {
        public string Type { get; set; }
        public double Value { get; set; }
    }
    public List<ModelData> ChartData1 = new List<ModelData>() { new ModelData() { Type = "S1", Value = 1 } };
    public List<ModelData> ChartData2 = new List<ModelData>() { new ModelData() { Type = "S2", Value = 5 } };
    public List<ModelData> ChartData3 = new List<ModelData>() { new ModelData() { Type = "S3", Value = 9 } };
}