When the user-provided metadata in a Gimp file includes XML special characters, the amersand is not escaped when exporting the metadata to a file (the export funciton with the metadata editor). The unescaped character makes importing the metadata back into Gimp impossible. (I have not checked behaviour with other XML special characters).
Addendum: experienced on MacOX 13.5.2 (22G91) (Intel)
I could not figure out how to report the bug on Github, or even determine whether the bug has already been reported.
First of all if you are not using the latest version of GIMP please switch to it. There was a lot of bugfixing in the metadata handling in the last versions.