It is definetely a bug. I have run into it allready just wasnt sure if i had to post it into forums too.
DFP2 Default File is not writing code to the page for the "default file". That is why it "doesnt retain the code". It actually does not exist so the behavior can find it.
If you click finish after u select a Default File no code will be written to your page.
You can confirm it by double clicking on the upload behavior and checking the default file field which is empty.
Allready submited a ticket, please guys this is a major bug, resolve it asap cause whenever u update a page using update.php or inserting a new row using a default image, the default image not being inserted in db and ofc on update.php the original filename is deleted from db since it doesnt contain the hidden form field with old data.
Please give us a minor release fast.