I'm guessing what you are experiencing is exactly what I was except I was only using a Pro 700/372/NAVII setup on the combine.  On my planting tractor it is a TMX2050.  I will try to look at what versions of firmware I have on my current NAV and Pro 700 on the combine.  For the past two years I've fed the 2020 GPS out of the NAV because I am using NTRIP RTK corrections.