The problem happens when enter a period . , single ' or " double quote on a new line.
The problem will be fixed in R3 2019.
The temporary solution is to apply the following override:
<script> var browser = Telerik.Web.Browser; Telerik.Web.UI.RadEditor.prototype._shouldIgnoreKey = function(e) { if (e.ctrlKey || e.altKey || e.metaKey) { return true; } else { var keyCode = e.keyCode; var k = Sys.UI.Key; var INSERT = 45, DELETE = 46; if ("code" in e && /^Digit/.test(e.code)) { return false; } if ("key" in e && /^(?:Digit|!|#|\$|%|&|\()/.test(e.key)) { return false; } if (browser.ie && keyCode >= 33 && keyCode <= 40) {//Shift+1, Shift+3, Shift+4, Shift+5, Shift+7, ... return false; } if (keyCode === DELETE && e.key === ".") { return false; } if (keyCode === k.right && e.key === "'") { return false; } if (keyCode === k.pageDown && e.key === "\"") { return false; } switch (keyCode) { case k.enter: case k.backspace: case k.pageUp: case k.pageDown: case k.end: case k.home: case k.left: case k.up: case k.right: case k.down: case INSERT: case DELETE: return true; } } return false; } </script> <telerik:RadEditor ID="RadEditor1" runat="server" EnableTrackChanges="true"></telerik:RadEditor>Please test it and let us know if the problem still persists.