Is it possible that in the target folder for the export where the index.php file is, there is also an index.html file? A simple explanation is that there is and that is the individual file which is being served up instead of the php version of the page.
If so, delete or rename or otherwise remove the index.html so that the index.php is served up.
Scenarios pivoting on the contention between php and html versions of pages and the fact that RW won’t remove already published/exported resources come up all the time.