I came across the same issue. According to Sencha support, that is the intended behavior:
This is not really a bug. The bind config is used to synchronize values between views and by default it does not publish the value when the field is invalid. So if you set allowBlank to false and the field is empty, this value will not be published to the viewModel.
Luckily I work on the same team as krullj and he mentioned having the override that would fix it. But since I've been told this is the expected behavior, I am wondering what the status of the bug ticket "EXTJS-15302" is.