I have done some testing with this and it seems that there can be issues with the page referenced in the action if you do not use the folder icon to choose the page. Whenever I create a form like this the validation code is correctly added to the action page and the validation code on the form looks correct. I can then update the action of the form after the fact to use the root relative path to the page.
I'm curious if you ever have this problem when you use the folder icon to select the action page. Currently it seems that the issue is just not using this folder icon to select the page, if you can add anything to this or confirm if this is not the case please post back let us know.