osmium export unexpected osm_ids

Hello

I've been exporting .osm files to .geojson using the osmium-export tool. I'm trying to keep the osm_ids(including the osm_type) so that we can lookup the osm objects downstream(e.g. through nominatim: https://nominatim.openstreetmap.org/ui/details.html

I'm using the '--add-unique-id=type_id' command to include osm_ids, so my command is:

osmium export --add-unique-id='type_id' -o tmp/processed-data/planet-ports-231009.geojson tmp/processed-data/planet-ports-231009.osm --verbose

This produces rich objects and reading the output into a geopandas dataframe does indeed produce an 'id' column as expected.

According to the docs:

Or the TYPE is type_id in which case the ID is a string, the first character is the type of object (‘n’ for nodes, ‘w’ for linestrings created from ways, and ‘a’ for areas created from ways and/or relations, after that there is a unique ID based on the original OSM object ID(s).

However, when I look at the output, there are 2 entries with the name 'Heysham Port' (I'd post screenshots here but I'm not allowed) and they represent seemingly the same geographical object, a port in the UK. These 2 objects have different osm_ids in the .geojson output: w974090785 and a1948181570.

I'm able to lookup the former on nominatim: https://nominatim.openstreetmap.org/ui/details.html?osmtype=W&osmid=974090785 But the latter is nowhere to be found, neither as node, relation or way. (tried n1948181570, r1948181570 and w1948181570) I might be misunderstanding this part of the docs: based on the original OSM object ID.

My question is, can I tell osmium-export to keep the original id? Alternatively, how could I match the true osm_id from the object of type 'a'? (Since I might prefer those objects, there are more of them)

Thanks for any help!