If I understand correctly, the correct way of dealing with expression maps shared between different documents is to import them in the document in edit. If you make a change that should be shared with all your other documents, you have to export the edited expression map, then import it again in the other documents, after having deleted the older, outdated map.
I would like to see a different method: expression maps are actually stand-alone files, to which a Dorico documents read the programming when you select the map. If you edit the external expression map file, you can select the map again in a Dorico file, and all the new settings are immediately updated in the Dorico document.
This will allow to preserve the existing programming (from which you can still export a different expression map), and at the same time be very fast in loading the updated settings from a shared expression map file.