Sadly, metadata as stored in a .kra file as well as metadata exported/imported via .png and .jpg has problems.
Here is an old topic related to this:
Meta data not saved
Notice the statement by the lead developer in that topic:
In general, the metadata system in Krita is pretty old and suffers from having started out as a research project, back in 2007. I’m not surprised there are bugs…
Another:
Author's name
Another:
Exif data and how to save them as jpeg?
The manual actually suggests using an external application to view/edit metadata:
Images for the Manual — Krita Manual 5.2.0 documentation
You can view/edit metadata using Layer → Edit metadata
However, there are problems with reliable storage of it, as you have seen.