Follow me sudden height increases

Down my local field today and was having a play with follow me, first time round I set it up for terrain follow and was getting height increases of about 30 feet out of nowhere on a level open field, after this happened, I set up the follow me parameters again but without the terrain follow option and no such problem,  I've managed to pull the appropriate section from the flight logs and saved it as a mission

https://app.dronelink.com/martin-reading/kingsmead/plan/xFsKNW9EkX3TvhgcGqV0