Does she get disciplined for these behaviors? If so, how?
I wonder if some of these behaviors began while residing with her bio parents as a survival coping mechanism.
Is she in mental health counseling? It has been my experience that foster and adopted kids usually have a lot of emotional issues typically brought on by their previous family situation and the inability to cope with their new family life. They usually have significant separation/loss issues. It is around the age of 11 when these kids tend to grasp the dynamics of their situation and begin to respond negatively. Genetic factors could also be at play. If she isn't, I would strongly advise that you consider this option.
It is pretty common for kids who have been in foster care to have a strong scarcity consciousness about food. In institutional settings, they will sneak it and hide it somewhere they can get it, even if they are not hungry and even if there seems to be enough availability. It is a response to the traumas of their lives. As MarkLakewood says, is she in counseling? She needs assessment and empathy, not anger and lack of understanding. A good counselor could also walk you through what to expect of someone who has been in foster care.
What kind of situation did she come from before you or was she taken from? Sometimes if the child was in a situation where there wasn't enough food or she had to eat from a dumpster it can be hard to adjust even years later. A part of them stays that way for a while I think for protection. It's psychological but just try reassuring her that she doesn't have to steal food or eat from a dumpster bc she will always have food. Therapy could help her top if she's not already going and if she is then try mentioning it to the doctor so they can try to help.