This quiet alone-time play is not a bad thing. In fact, I see it as an important development for her. When she first came to us, it would have been unbearable -- nay, impossible -- for her to spend so much time alone. Too many fears and anxieties. Now she is comfortable with herself, and comfortable in our house, and she is developing a new skill, the ability to play alone.
Community is important; connection to others is important. But there's also something tremendously valuable in being able to enjoy your own company. She played by herself for a while, came out and connected with me for a while, then went back to the world of her imagining. I think she is striking a good balance.