Chapter description is not exported during the book export (PDF) #1465

Describe the bug
When I have a chapter with a manually written description, I can't see the description in the PDF file if I export the whole book. If I just export one chapter, I can see it just fine. I don't think that this issue is related to the PDF exporting engine (I use wkhtmltopdf) as I see empty <p></p> instead of <p>DESCRIPTION HERE</p> in the intermediate HTML file. Also this issue is reproducible on the demo site.

Steps To Reproduce

1. Create a new book named "Test"
2. Create a chapter named "TESTING" with a description "HURR DURR TESTING STUFF"
3. Export the book to PDF
4. Search for "HURR DURR" without any luck
5. Export the chapter to PDF
6. Succesfully search for "HURR DURR"

Expected behavior
A manually written chapter description should be visible in the PDF file when exporting a book.

Your Configuration (please complete the following information):

• Demo site (24 May 2019)