I couldn't spot an obvious cause for the described problematic behavior.
I have created a basic sample project based on the provided code. I have switched the alert to console.log() in the event listener for more convenient tracking of the results. On my side, it behaves as expected. Please give a try to the sample to see how it works on your end. (To run it you will need to place the Telerik.Web.UI.dll and Telerik.Web.UI.xml in the bin folder.)
It would be very helpful if you provide us the steps to replicate the issue with the sample.
Five days of Blazor, Angular, React, and Xamarin experts live-coding on twitch.tv/CodeItLive, special prizes, and more, for FREE?! Register now for DevReach 2.0(20).