SUMMARY
Taken from https://www.reddit.com/r/krita/comments/e3lkds/cant_get_author_information_to_save_to_file/

Author data cannot be saved/loaded properly when exporting or saving to PNG or JPG files.


STEPS TO REPRODUCE
1. Edit author data in Configure Krita
2. Settings -> Active Author Profile -> select some profile other than anonymous.
3. Create a Krita document. Save it as a kra file.
4. File -> Document Information -> Author. 
5. Export it to a png file. Check "Save metadata" and "Sign with author profile data."
6. Open the png file.
7. File -> Document Information -> Author.
8. Modify the png file and save it in-place. Check "Save metadata" and "Sign with author profile data."
9. File -> Document Information -> Author.
10. Close the png file, and reopen it.
11. File -> Document Information -> Author.
12. Repeat 5-11 with jpg.

OBSERVED RESULT
Only in steps 4, 9 can I saw the author information.

EXPECTED RESULT
We should be able to see author information in 7 and 11 as well.

SOFTWARE/OS VERSIONS
Windows: 10
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 5.12.3 

ADDITIONAL INFORMATION
For the JPG case, I am able to see the author filled in in the properties dialog of Windows Explorer, so it could potentially be a loading issue, rather than a saving one.