GoBotany only treats 2 of the 6 flat-topped goldenrods in Flora of North America.

Several other authorities call 'em Solidagos:

FNA: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=316945
The Plant List: http://www.theplantlist.org/tpl1.1/record/gcc-755
ITIS: https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=36296#null
Michigan Flora: http://michiganflora.net/genus.aspx?id=Solidago
Weakley 2015: http://www.herbarium.unc.edu/flora.htm

They can always be swapped back, but for now the two taxon entries should at least be combined into one.