When I add a new event to Calendar, what I type reverts to "New Event" when I press Return. Why?

When I add a new event to Calendar, I type the event's name into the field, change the time, add any notes, etc. When I press Return, the new event briefly shows on the calendar as what I typed, but most of the time, it reverts back to "New Event." I have tried to change my sequence of what I type and sometimes press Enter instead of Return, but there doesn't seem to be a pattern to this behavior. It also doesn't happen all the time—just most of the time.

This is extremely frustrating because if I don't notice it right away, I have no idea what the event was.

Any suggestions?

I am using OS X, 10.9.5.