THis fixes it
<div class="custom-control custom-switch">
<asp:CheckBox runat="server" ID="custom_check"/>
<label class="custom-control-label" for='<%: custom_check.ClientID %>'></label>
</div>
protected void Page_Init(object sender, EventArgs e)
{
custom_check.InputAttributes.Add("class", "custom-control-input");
}