Looks like Ruiz is playing hard to get in terms of the venue, I doubt his rematch clause stated he needs to fight at any particular venue, so he's obviously going to try and stand his ground and get the rematch held in the states where he will have the home advantage.
However, as the champion I would say that he is well within his rights to play the A side, and Hearn should budge to him if necessary and stage the fight in Madison Square Gardens again. I don't think the fight will go the distance to see any bias on the scorecards, but Joshua has a wrong that needs to be made right in the place it first occurred.
At least the fight is not going to be held in the English soil, I have seen a lot of Joshua videos on Youtube analyzing why he lost to Ruiz
this is one of the videos
https://www.youtube.com/watch?v=p20mo8Bed-I to cut the video short he underestimates Ruiz, but if he will come prepared I'm pretty sure Ruiz will also come prepared.
I agree that Anthony Joshua probably did underestimate Ruiz, however, even with that underestimation Ruiz still completely stomped him.
It wasn't like Ruiz outclassed him in a technical boxing match, or got a lucky shot in. Ruiz is a counterpuncher and Anthony Joshua tried to get into too many swing ups.
Every time they went for a swing up Anthony Joshua got manhandled. So long as he doesn't make that mistake in the 2nd fight, and instead keeps Ruiz at the end of his jab, Joshua can certainly win.
However, based on the fashion of his first loss, I think AJ will probably lose again unless he fights a technical masterclass fight.