The RadMultiSelect implementation has one error that WCAG 4.1.1 parsing is catching. There is a duplicate span with the same id.

Found in the page source of the demo page (similar to how I implemented as well):

https://demos.telerik.com/aspnet-ajax/multiselect/virtualization/defaultcs.aspx

<select name="ctl00$ContentPlaceholder1$RadMultiSelect1" multiple="" id="ctl00_ContentPlaceholder1_RadMultiSelect1" class="RadMultiSelect RadMultiSelect_Silk" style="width:400px;">
<span id="ctl00_ContentPlaceholder1_RadMultiSelect1_WebServiceCDS" style="display:none;"></span><input id="ctl00_ContentPlaceholder1_RadMultiSelect1_ClientState" name="ctl00_ContentPlaceholder1_RadMultiSelect1_ClientState" type="hidden" />
</select><span id='ctl00_ContentPlaceholder1_RadMultiSelect1_WebServiceCDS' style='display:none'></span>