There is a potential bug in the Google People API that started recently where updates to contact birthdays are no longer updated/captured. This results in a loss of data where the birthday value is ignored, no error is returned and the contact birthday is blank on the Google contact entry.
For years the following code has been working flawlessly. I can work around this issue by using the Text (free form) field to set the birthday but this is less explicit vs. using the formal and documented Date property.
Sample code that has worked for years and recently stopped working.
person.Birthdays = new List<Birthday>() { new Birthday() { Date = new Date() { Year = 2000, Day = 10, Month = 1 } } };
Work around but not ideal...
person.Birthdays = new List<Birthday>() { new Birthday() { Text = "10/1/2000" } };