In case this helps anyone, the achievement behavior seems a bit wonky. I finished the “into the storm” achievement a year ago, but noticed it changed the completion date to the day the patch dropped, but only on the character that I first logged on with that had all the credentials (not my main). It seems like Storm Chaser and Into the Storm were the only account wide sub-achievements. Elemental Overload and Stormed Off are greyed out on alts that did not complete them. All other characters, even if they had the credentials, who did not log in first have the old achievement date. I seem to have received the mount in the mail on the character I first logged into on the first day of the patch that was able to re-trigger the achievement via the credentials and has the new achievement date.