The screenshot below is from a phone, but I have confirmed this occurs on a desktop browser as well:
I tried to add the butterfly genus Sea to my favorite taxa (https://www.inaturalist.org/taxa/334472-Sea )
It does not appear in the list when I try to type it in the box for entering a favorite taxon:
I don’t know how many other taxa might have the same issue. I found a workaround because Sea is monotypic, so I was able to enter the species instead of the genus, but that might not work for other taxa and the option to select from any taxa, not just the first [ X ] returned, should really be an option.
I just checked on the Place subscription search and it also seems to return a limited amount of results with no option to see if there are more present in iNaturalist.