PDFs lose data when saved from email attachments in Preview

When saving PDFs opened from email attachments, the data is disappearing. The formatting remains but the details go when I reopen the file on Preview from the folder (they also show as blank in the Finder folder thumbnail).

They seem to be documents created using accounting software - so a form template which is populated by the sender - and they open fine from the email itself. I have saved lots of the same type of file before and they have been ok - and still open now.

When I save the documents in a folder, the details disappear (leaving just the logo and some formatting). What's going on? Is there some sort of conflict or layers issue?

I tried downloading another PDF viewer (as an alternative to Preview) which was suggested on another thread, but no joy. I was running an older version of MacOS also, but updated and still no progress. I also tried 'printing' the document and saving it again as a PDF (also suggested on another thread to 'flatten' the PDF) and nothing changes.

Any suggestions welcome.

Running on MacBook Air 10,1 with M1 chip and Sequoia 15.7.3