Designated Survivor’s first showrunner, Amy B. Harris left the ABC program after the pilot got picked up to series, reportedly due to creative differences.
For instance, Feldman is reportedly working out an overall deal that will keep him with ABC for at least two more years.
Deadline notes that with Jon Harmon Feldman now leaving, there’s no word as to why the showrunner is departing, but that there’s certainly no ill will with the network.
Again, that’s simply a possibility not a probability.