The event text gets HTML-encoded (as a protection against XSS attacks) and then it is treated as HTML. That means ASCII line breaks will be ignored.
If you need to insert a line break, you'll have to use the BeforeEventRender event and build the HTML manually.
Example:
protected void DayPilotCalendar1_BeforeEventRender(object sender, BeforeEventRenderEventArgs e)
{
e.Html = HttpUtility.HtmlEncode(e.Text) + "<br/>" + "Another line";
}
Don't forget the HTML-encode the input text (e.g. using HttpUtility.HtmlEncode() like in the example above).
See also:
https://doc.daypilot.org/calendar/event-customization/