Dude, you'd do the same thing xD
"Man, I don't want to fight the super duper extremely stupid strong dude, so instead I'ma take the dude fighting with a DC character." << Sure, this is showing that she may prefer to have fought Deathstroke first, but no rules were broken xD
When a Beast is lost, changes host, stripped, any challenges/challengers have to place their challenge again o.o It happened when I got both the Six and Five Tails and countless other times over the years.
And a Bijuu host doesn't need to accept a challenge until after their grace period BUT they can if they want to. Meaning, you could challenge me twenty times during my grace period and I could say no each time and then a random acad comes and challenges me and I say sure, toss him on my list, and now he's my first fight once my grace period is up. Sure, it may sound sucky, but that is how it works.
That's exactly this situation. You asked, she said no, and then Deathstroke asked, and then she said yes.
She hasn't broken any rules, or done anything to even be put on a 'Three Tails Trial' xD Sure, you could say she played favorites, but she played favorites without breaking a single rule >>
So, yeah <<