If there is no way to change this in your software somewhere, you can do (make a backup copy of your mxml file) the following:
- open the file in any text editor (provided you have exported as uncompressed mxml that is simply a text file) and look for this part:
<scaling> <millimeters>84</millimeters> <tenths>400</tenths> </scaling> <page-layout> <page-height>519</page-height> <page-width>448</page-width> </page-layout> </defaults>
- export a proper formatted score from Dorico as mxml to see what this file has in place of these parts. Try transplanting that part from the proper file to the “bad” one and see if this helps.
Tedious, but if there’s no other way, should work