Hello Bevan,
In order to prevent flattening of form fields, you will need Adobe Acrobat for the same.
Open the PDF form and go to File> Save as Other> Optimized PDF
In the PDF Optimizer dialog box, click at 'Discard Objects' and uncheck 'Flatten form fields' option under Discard Objects Settings (screenshot shown below):
Now try and let me know how it goes.
Regards,
Anubha