I hope this is the right place to report bugs, if not, let me know where to go, but as the title says, Defaults() is broken.
As this other topic also shows, it's broken in more than one way, but it is acutely broken when referencing a "Date and time" type field where the default value is set to "Today's date" - it is simply not returned whatsoever. Put a "Date and time" field in a SharePoint list, set it to a Default value of "Today's date", and it will not show up even if it's explicitly referenced by Defaults('List').Field. It does work with a fixed date.
I suspect, but I am not willing to test, that this will probably affect default values which are similarly dynamic, i.e. calculated.