This bug is related to the text shown on top of the message groups. The text shows the time + the author's name. For example: "12:30PM,John Doe".
Add a space between the time and the author's name. It should be "12:30PM, John Doe", instead of "12:30PM,John Doe".
this.Chat.AddMessage(new TextMessage(new Author(" John Doe"), "Hi", DateTime.Now.AddDays(-3)));