The cause of this seems to be a bug in the merging of synthetic names during combined index construction. The synthetic parent taxon inherits the scientificNameID, which is also used as a taxonID for the original taxon. This causes an erroneous lookup in the identifier index.
See https://github.com/AtlasOfLivingAustralia/ala-namematching-service/issues/53
The cause of this seems to be a bug in the merging of synthetic names during combined index construction. The synthetic parent taxon inherits the scientificNameID, which is also used as a taxonID for the original taxon. This causes an erroneous lookup in the identifier index.