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>
Thank you for reporting this accessibility issue! I verified it and logged it for fixing for the upcoming in October R3 2023 release. I created a bug report item where you can follow the item updates - https://feedback.telerik.com/aspnet-ajax/1621489-radmultiselect-wcag-4-1-1-parsing-error-duplicate-dom-control-with-the-same-id.
Your Telerik points have been also updated.