For an example, start ThreePartsDate.yml, enter info into the high school graduation date, and only partially complete the Birth date field. When you try to continue, there will be a validation error on the Birth date as expected, but the high school graduation date no longer be filled, which is a frustrating experience for the user.

I'm not sure why this is: we have a correct default_for function, which the documentation on CustomDataTypes suggests is the right function to implement for things like this, and it seems to work correctly, as you can fill in both correctly, proceed, and then go back, and the values are correctly there. So it too seems like an upstream issue.