I have an ICS calendar that is drawn from a website. I have noticed that if the description in a calendar event is too long, it hides the title in the calendar widget.
For example:
In the first screenshot, the calendar event includes a longer weblink to a web calendar in the description field. In the second screenshot, the description is removed, and the title is visible again. As far as I have tested, the description field is the only attribute that affects the widget's title field.