When Miranda's loyaly mission becomes available, hold off on doing it until after Jack's has been unlocked. After doing Jack's mission, take Jack's side in the argument. Next do Miranda's mission and her loyalty will be back.
Yeah, after playing a little more with Genesis created characters, I'm more convinced that something is wrong with it. I frequenly have problems unlocking Paragon/Renegade options that I never have with other characters. For example, the quest to earn Zaeed's
loyalty, I did that one as my first mission with an imported Shepard with about a 20% paragon Score and it worked fine. I tried the same mission with a Genesis Shepard with a 90% Paragon, and it was greyed out. After trying a few different times, I finally
had to give up on the Genesis tool, which is a bummer because I really thought it was a cool idea. I wanted to have a Paragon Fem Shep loyal to Liara, but apparently it's not to be. I would play through ME1 again, but just the thought of driving that damn
Makko again is just too much to handle without going insane.
That's incorrect THE CHAOS KING. The arguement between Jack and Miranda occurs
after completing both of their loyalty missions.
Still, it's never been a problem for me as I usually have enough Paragon/Renegade points by the time I start doing all the loyalty missions, so I've always been able to get the Paragon dialogue option in all of my previous playthroughs.
Deaths Head is right, the OP's advice will not work because the argument doesn't trigger until after you have completed both loyalty missions.
I have noticed a frustrating bug with this though, from what I have seen in my testing, you cannot secure the loyalty of both characters if you're using an imported character that was made with Mass Effect Genesis. I have played this game many times with
an imported character and had no problem, my new character from scratch was also able to settle the argument. Any character generated with Genesis however cannot solve this conflict regardless of how high the Paragon/Renegade score. I have had the same problem
trying to secure Zaeed's loyalty after taking the Paragon Option to save the factory workers. With a Genesis Created character, there is absolutely no way that I have found to unlock the paragon option to keep him loyal.
With other characters, I simply do Miranda's shortly after it becomes available and do Jacks nearly last. By that time, I have more then enough Paragon/Renegade to solve the problem.
I've noticed some inconsistencies with confrontations using Genesis created Shepards. But I haven't had a problem with Jack and Miranda. I have a problem with Tali and Legion, though I've never had a problem resolving their conflict using an imported Shepard.
On the other had, I always have the choice of Samara or Morinth using a Genesis created Shepard, but I rarely have the choice with an imported Shepard. With Zaeed, I always wear the Inferno armor thinking that gives me some kind of edge when doing the paragon
option on his mission and I always get the charm option to secure his loyalty no matter the type of Shepard I use. So I just think the charm/intimidate algorithms get thrown off when you use a Genesis created Shepard.