I think I might've found the issue... so, near the start of the "My Info" section, it asks, "Did either of you make money in any other states?"
I answered that "Yes" initially since all of my income comes from an employer in CO, but I should've answered, "No," since I never physically worked in that state.
I think that some additional explanatory language should be added to that section, to help guide the huge wave of remote, out-of-state employees that now exist in the country. (CO's own tax website is similarly confusing on the subject.)
