So this one is a bit weird. I've had a few battles with Miao Ying where she was unable to transform into her dragon form (it had a red X through it and was greyed out like an ability that used all of its charges.) I *think* it might have something to do with her having been on the ground in human form for too long without transforming, since in the battles where I made sure not to delay transformations it worked perfectly.
I've encountered this particular bug both when I don't transform her at all, and after transforming from a dragon into a human. It's only a lock when she's a human, transforming from a dragon into a human has not had this issue.