Mavic 3E - Terrain Following

I purchased the Mavic 3E with the RTK module and was trying to use the terrain following feature but cant seem to get it to work. The terrain I am working in is very steep and narrow with the height difference between the ridges and valleys exceeding 400ft. The mapping mission starts off fine, I had it set to fly 300ft above the terrain, but as it progresses up the valley it claims it has hit the 400ft ceiling and does not progress with the mission. Has anybody tried using the terrain following feature in rugged terrain? It seems like it is using the take-off elevation as it's basis and not the terrain from the DSM.