Pressbooks tells Prince XML what PDF flavor to use. Print PDFs use PDF/x-1 standard and part of that standard is that images are not allowed to be transparent.
Here’s a discussion about it from the Adobe Forums:
https://forums.adobe.com/thread/2369506
PDF/x standard are meant for exchange where all the PDFs information (styling/fonts) is embedded in the document, which is why Pressbooks uses it for the print standard. The only PDF/x variant that supports transparency is PDF/x-4.