PowerPoint drops hash characters in hyperlinks when saving
You add a link with a bookmark, like this, for example:
The link works when you test it but after you've saved and reopened the file, the link no longer works correctly, and when you check it, you find that PowerPoint has dropped the bookmark portion (the bit after the hashmark).
If you insert the hyperlink as you normally would but replace the hash (#) character with %23, PowerPoint retains the link when reopening the file but the next time you save it, it's lost again.
One workaround is to save as PPT (97-2003) format instead of PPTX/PPSX/PPTM. This has its drawbacks as well, but if the links are the most important thing, it might be the best nearly-a-solution.