You can provide custom HTML that will be used for event content using BeforeEventRender event handler:
https://doc.daypilot.org/calendar/event-customization/
Your event handler could look like this (C#):
protected void DayPilotCalendar1_BeforeEventRender(object sender, BeforeEventRenderEventArgs e)
{
e.Html = e.Text.Replace("\n", "<br>");
}