This is an issue about exporting / importing scripts that are not in the default package.
Can't be imported from a workspace from a previous version (didn't work to import in Studio 7.8.3 from a 7.8.0 workspace);
Can't be exported as a bos archive (see "steps to reproduce").
The class works fine in groovy expressions within the process.
Leave the package name empty at the Groovy class creation. Despite the warning "The use of the default package is discouraged", it works well in groovy expression (even better : static methods appear in user defined functions), and no more problem about exporting these classes. Although it seems to be a bad practice.