Does your daughter by chance have joint hypermobility? It isn't inherently bad, many flyers/gymnasts/dancers have it, but you might ask your pediatrician to perform the Beighton test on her (or you can Google it). If so, she may need to focus more on strength training to progress in tumbling and protect her joints. With hypermobility, flexibility classes aren't stretching muscle, they are stretching ligaments and tendons that are already too flexible for their own good.
My youngest's PT had us get her tested for hypermobility, and tested positive. I ask about this because she, also, was undercutting her BHS (feet beginning and ending relatively in the same place) because of her incredible back flexibility and hundreds of spotted reps putting it to muscle memory. My youngest was a base, however, she gravitated to flexibility classes and exercises because flexibility was inherently easy for her. Her PT put a stop to flexibility classes and had her focus on strength training. If your daughter has joint hypermobility, the strength training will only serve to protect her joints from partial and full dislocation, and help with the strength needed to get the proper technique in her tumbling.