Checkbox buttons are missing when ToggleType='CustomToggle' in Lightweight rendering. The issue is introduced in 2017 R1 release.
Code to reproduce:
        <telerik:RadButton runat="server" ID="btnEnableTrace" RenderMode="Lightweight"
            ButtonType="ToggleButton" ToggleType="CustomToggle">
            <ToggleStates>
                <telerik:RadButtonToggleState PrimaryIconCssClass="rbToggleCheckbox"></telerik:RadButtonToggleState>
                <telerik:RadButtonToggleState PrimaryIconCssClass="rbToggleCheckboxChecked"></telerik:RadButtonToggleState>
            </ToggleStates>
        </telerik:RadButton>