If the cache metadata file doesn't exist yet, and if the set() function is called first (before get()), the key/value isn't saved.

So a code like:
gedit_metadata_manager_init (...);
gedit_metadata_manager_set (...);

The problem is located in this code in set():

> if (!gedit_metadata_manager->values_loaded)
> {
>        gboolean res;
>
>        res = load_values ();
>
>        if (!res)
>               return;
> }

load_values() returns FALSE if the file doesn't exist.