Git detached head after pull/push fail
So the issue was not a detached head, as I anticipated. Like Matthieu mentions, my mistake was in thinking that I was up to date because the origin repo was reading that it had been committed/merged.
Because Heroku is a separate remote, seeing that my pull/push for the origin remote completely missed the important question (hence my confusion): is the Heroku remote updated?
To do that, I ran git push heroku master -f and that brought the second remote up to speed, and we're back in action.