Hello,
I understand the issue, let's explain some details, when you create a default value for your custom field, this will be add "by default" on each NEW product, you will create from now.
Because for older product the custom field is unable to add it in your data base.
But as you can see the input is display in your product backend, ready to be fill, and so you can add this same default value.
If you have a lots of old products, I think you can use Massaction to "update" these products with this default value.
Hope this solve your issue.
Regards,