This was a balance issue already in Civ5; in Civ 6 it is simply a bit harder to get the +1 range promotion to artillery units, but it is nonetheless a balance issue again. AI is unable to defend militarily efficiently against the human player and the +1 range makes things even worse, because it allows the human player the opportunity to attack cities without having its artillery units attacked. Even on an open battlefield artillery promoted with +1 range are lethal because AI doesn't take in account the increased range nor prioritizes them as targets. For balance reasons i think this ability should be removed or at least there should be a defensive building which increases city defense range to 3. In the latter case though the balance issue would be only partially resolved for the already listed reasons.