If you set Title, Caption, and Copyright to be non-blank strings and then later clear those fields and do Metadata > Save Metadata To File, the corresponding IPTC fields in the file will contain the previous strings rather than be empty.
1. Select a JPEG and in the Metadata > Default panel set Title, Caption, and Copyright to be "XXX".
2. Do Metadata > Save Metadata To File.
3. In the Metadata > Default panel, delete the contents of the Title, Caption, and Copyright fields.
4. Do Metadata > Save Metadata To File.
5. Do "exiftool -iptc:objectname -iptc:caption-abstract -iptc:copyrightnotice file.jpg" and see that IPTC:Caption-Abstract those fields still contains "XXX".
6. In LR, do Edit In > Edit In Adobe Photoshop > Edit Original.
7. In Photoshop, do File > File Info and observe those three IPTC fields still contain "XXX".