I just tried this myself. After exporting from Lightroom Classic, I used three other applications (desktop properties, Apple Preview, and Adobe Bridge) to verify that all the metadata was actually in the file. It is. But after uploading to a Facebook photo gallery, Facebook does not display the Title or Description of the image.
Because Lightroom Classic did its job and included the metadata in the exported file, it looks like this problem is in Facebook. Over the years, it has been observed that how Facebook imports photo metadata can change at any time. It has changed back and forth in the past.