Search picks a species instead when searching for genus - "Warning an unresolved homonym has been detected"

[timhicks-ala]

A user's reported an odd issue searching in AVH. If you search for "setaria", it brings up a particular species of it instead of searching for the genus: https://avh.ala.org.au/occurrences/search?q=taxa%3A%22setaria%22
For others, eg acacia, it correctly searches for the genus: https://avh.ala.org.au/occurrences/search?q=taxa%3A%22acacia%22
I can manually search for just the genus by borrowing most of a URL from biocache, so the user has a workaround: https://avh.ala.org.au/occurrences/search?q=lsid:https://id.biodiversity.org.au/node/apni/7774108#tab_recordsView

This is not isolated to AVH, the same result can be seen searching directly from Biocache - it is likely masked there because users will search first from the homepage and click the genus in BIE search results. https://biocache.ala.org.au/occurrences/search?q=taxa%3A%22setaria%22#tab_mapView

Digging into this, the problem is that the search for setaria returns Warning an unresolved homonym has been detected. whereas acacia does not. This means the second level of matching takes place, a common name search. This is why taxa:setaria matches https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/7774102
This is the behaviour of ALANameSearcher.getGuidsForTaxa in ala-name-matching-service:4.2.

Raised in https://support.ehelp.edu.au/a/tickets/193803