Mavic Pro measures displayed altitude relative to the take-off point barometric pressure. My original assumption was that it used GPS computed altitude but that is incorrect. The Mavic has a built in limitation of no more than 400' above take-off point. Normally not a problem but when flying in mountainous areas, the 400' can be exceeded without going too far horizontally and that precludes getting a picture of what's on the top of the mountain of interest.
The smart follow modes would have gone up and over the hilltop and down into the ravine at a fixed position relative to the tractor and thus would have followed the terrain if you had locked the drone on the planter before he went up and down the hill. 
I have seen reports of folks who have been successful in "gaming" the altitude restriction by putting the drone in a case that can be slightly de-pressurized and then turning it on. Fools the drone into thinking it is higher than it really is and thus gets around the altitude restriction to some degree.