Made two flight plans yesterday on PC, uploaded and executed the first on a Phant 4 Pro - started - all greens - took off. Drone flies up - camera points down, and the drone flies away - far away from planned waypoints (I managed to take over and recover the drone) - Made a new flight plan on site - tried again - same story…
Did you calibrate the compass beforehand.
Always a good thing to do.
Also there could have been interference.