Remember that the use of element boundaries (whether shown as square brackets or as tags) moved from a document property to a session property (stored in maker.ini) in FM 2017. Updating your book shouldn't change the property. However, if the session property is set to view tags and you open any book component, you will see the tags. Furthermore, it seems that if you print a single document, FM asks if you want to print with tags showing. If you save a single document as PDF, print a book, or save a book as PDF, there is no prompting.
I can't duplicate this. What type of files do you have in the book?