If this is the case it should be noticed in the values just looking at them. The OP stated that the ground values and image values are what they expected. If the ground was ortho and the images ellipsoid, it should be obvious looking at the values as the image values would be too low compared to the ground if the ground was using ortho. But we are kind of shooting in the dark here.
My thinking was this.
He stated that the GPS heights (Assuming he meant GNSS Rover) were 58m, which is the orthometric elevation for that area shown on the RC Plus 2.
All DJI mapping drones with RTK use an ellipsoid elevation when receiving corrections, so that's what's in his image metadata.
He says his drone data (dem I assume) is reporting the ground as ~ 10 meters. The geoid undulation in that area is ~ 46 meters.
But like you said, a csv and some sample images would answer any question.
OP Dave or myself could help out with 6 images from 2 flight lines overlapping each other and the csv with rover data.