Try cleaning the cache manually for InDesign. Which OS you're on?
The cache location for Mac : Macintosh HD/Users/Username/Library/Caches/Adobe InDesign/Version 8.0/.
The cache location for Win: C:/Users/username/AppData/Local/Adobe/InDesign/Version 8.0/
In both cases you've to drag the en_US folder to the desktop to clean the cache completely.
It will always be the name of the file until you change it. After that,
you're stuck changing it every time.
Changing it back inthe export dialog and saving the .indd file after the export should fix it, or export to .idml/.inx as appropriate for the version should reset it as well.
@wolfie585 – in the future there are some rules you can follow to avoid this problem alltogether:
1. Before exporting to PDF, save your file
2. Then export, choose a file name as you wish
(then a * in front of the file name in the window tab will inform you that your file has changed!)
3. Revert to last saved state
4. If you get back to export to PDF, InDesign will not remember the name you have chosen in step 2
Hope that helps,