Problem with Vollkorn fonts in KDP (Amazon) books

I used Vollkorn 5.x, downloaded from Google fonts, in an Amazon book. When I uploaded the book to the layout checking tool everything looked fine. But when I downloaded the PDF proof, there was a serious issue.

All the italic small letter "a" were filled in (see screenshots).

I searched the KDP forums and found two other users with this issue. One commented that they had published books with Vollkorn on KDP before with no problems, but now had the issue I described. Because it once worked for the other user, I downloaded 4.x version from the Vollkorn website. I uninstalled the Google font 5.x version and installed the older 4.x versions - the problem disappeared.

**To Reproduce**

Use Vollkorn from Google fonts in a manuscript, upload to KDP and download the proof PDF. All the italic small "a" letters are affected.

**Expected behavior**

The italic "a" letters should render properly in KDP and other applications.

**Screenshots**
First Screenshot - Vollkorn 5.x
Second Screenshot Vollkorn 4.x

**Additional context**

I use a Mac running Sequioa 15.5, Microsoft Word, PDF rendered with Adobe Acrobat for uploading. The PDF I uploaded had no issues.