Telerik UI For ASP.NET AJAX R2 2018 SP1 (version 2018.2.710) Retail
Could you confirm whether you have upgraded to R2 2018 SP1 (2018.2.710)? This issue is resolved there. You can test this on our demos to confirm (video of the expected results is attached below): -ajax/searchbox/examples/programming/serverevents/defaultcs.aspx.
Telerik UI for ASP.NET AJAX R2 2018 SP1 (version 2018.2.710) Retail
This seems to work fine in our demos. I am attaching here a short video that shows my testing steps and another one that shows how you can check the actual version used on the page. Could you confirm if you see 2018.2.710? Am I missing something in my test?