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>