On Friday, Google updated its FAQ structured data guidelines to provide examples around the guideline that reads "all FAQ content must be visible to the user on the source page." The examples explain that it is okay to have the content in expandable sections.
Here is a screenshot of the new section that was added:
Google said it "added example use cases for the FAQ guideline about hidden content on the page. The user must be able to access the answer on the page, and clicking an expandable section to view the answer is a valid use case."
This is an important clarification for those that use FAQ markup on their pages.
Forum discussion at Twitter.