Thank you for looking into it! I will track the issue at the link you gave.
In my experimentation, I tried loading the original Dorico-generated PDF into Illustrator and surprisingly it showed up ok. If I Save As from Illustrator into a new PDF, the final result looked good in Acrobat.
Even more intriguingly, if I load the original Dorico-generated PDF into Acrobat, it displays and prints incorrectly as before. But if I merely use Acrobat to save to a different file, the resulting PDF seems to have the problem fixed(!)
So I guess a workaround for now is to load the generated PDF into Acrobat and re-save. There must be some subtlety in the way the original PDF is generated that Acrobat doesn’t like, but which it seems to fix in the process of re-saving. Weird.
Edit: Ok, it’s not as simple as that. I tried reproducing the “fix” and it doesn’t happen again. I will have to retrace my steps to figure out what I did to “fix” it.
Update: Arrrgh, I tried so many different ways to reproduce it, but could never make it happen. My current suspicion is that Acrobat and Illustrator share an engine underneath, and something I did in Illustrator had a side-effect of making things render differently in Acrobat. But it’s a pretty weak theory. Hopefully the Qt folks can figure it out!