End date cannot be reset to NULL, defaults to start date value

I have a date field that allows an optional end date. My field settings for default values on both date and end date is "No default value". If I save a node with a value in end date, it is impossible to reset the value back to NULL. I can save the node edit form with no values in end date, but it always saves end date with the value in start date!

Steps to reproduce:

1. Add a date field with optional end date and default values set to "No default value"
2. Create a node with date and end date values. Save the node.
3. Edit the node and remove the end date vales, untick "Show end date", and save the node.
4. Observe that the end date value is now the same as the (start) date value.

This is a bug because it prevents users from clearing the end date value, automatically inserting a value that the user did not intend to set.