Filliable PDFs opening as blank in Document Cloud via SPO

Our users download fillable PDFs from Government websites for our clients. A form was completed, uploaded to SharePoint Online and then when the user accessed it using Document Cloud, the fields were all blank.

I've managed to recreate this on my account, if I open it locally or "in browser" the content is there, but if I check it out and then open with Document Cloud, it shows as blank.

I have a full Acrobat licence, am logged in and have had a look to see if there's a setting that might cause this but am drawing a blank.