Only the page content appears in the HTML export

Steps to reproduce:

1. Create a page e.g. P1 and add some content
2. Create a child page P2 (P1/P2)
3. Create a child page P3 (P1/P2/P3)
4. Export as HTML page P1 with children
5. Extract the zip
6. Open the Index page
7. Open each page

Expected results: The exported pages are the same as the original ones.

Actual results: Only the first page that appears in the list is showing properly. The other 2 pages appear as the content of the page only - the left/right panels are missing, also the top menu, header and footer.

NOTE: HTML export works fine on 11.7