I'd log a bug in Redmine (https://redmine.pfsense.org) under pfSense Packages for this. Make sure to choose Status Traffic Totals as the category. There are a number of other bugs for this package also logged there. The package was created by a Netgate employee a few years ago when RRD was dropped as the graphing tool, but I don't think it's being actively maintained anymore.
Good to hear - I found this out also because I fiddled with my interfaces long ago and had the same issues, not a very intuitive fix but at least it works.