Reproduction steps:
Go to the Telerik demo at https://demos.telerik.com/aspnet-ajax/textbox/overview/defaultcs.aspx
In the Comment box, type "one two". Do not let the textbox lose focus.
Highlight "two" and start to drag it to move it before "one". The other text ("one") will disappear, and is gone forever, as far as I can tell.
It can be also reproduced with this configuration:
<telerik:RadTextBox RenderMode="Lightweight" runat="server" ID="RadTextBox3" Width="200px" EmptyMessage="Enter comment" TextMode="MultiLine" Height="100px" Resize="None"></telerik:RadTextBox>
We just released the Latest Internal Build 2019.2.814 of the suite with a fix for this issue and wanted to notify you about that.
You can check the release notes at https://www.telerik.com/support/whats-new/aspnet-ajax/release-history/ui-for-asp-net-ajax-2019-2-814-(nightly-build-2019-08-14) as well as download the Telerik_UI_for_ASP.NET_AJAX_2019_2_814_Dev_hotfix.zip installation from https://www.telerik.com/account/product-download?product=RCAJAX.