Thank you for writing to us.
Indeed, there is an issue with the filtering functionality in TreeView and I have already logged it to our Public Issue Tracking System. Please follow this link
to add your vote and also subscribe to its status updates. We will do our best to address the issue in one of our next releases. I am not able to provide you with a workaround for the time being, due to the nature of the issue.
the Telerik team