We experienced something similar when my daughter was very young. She was eventually diagnosed with a social anxiety disorder called Selective Mutism. She would only speak to immediate family, and as she got a bit older, she would speak to a few of her friends. When any other adults tried to speak to her, she would put her head down, or whisper in my ear for me to respond to them. I never realized how bothered adults would get when a child refused to speak to them! Some of them would even make a point to bring her candy, but it made no difference because she wasn't ABLE to speak to them. I had to learn through therapy not to try and make her say thank you to people for bringing her things because she wasn't being defiant, she truly had no control over it.
She would have horrible meltdowns. It must have been so frustrating for her to go through an entire day not being able to speak to people, so when she got home to her safe place, she would completely lose it.
I'm not saying this is what your little girl has, but kudos to you for having her evaluated. Early intervention made all the difference for my daughter. Her social anxiety was severe. Trust me, I cried many tears over it because as exhausting as her meltdowns were, and as much as her tantrums tried my patience, my heart broke for her. Today, she is 22 years old, is in her senior year of college, and .... is a social butterfly with tons of friends. She even works as a waitress! I never thought I'd see the day, because one of the last things she was able to do (even as old as 12 or 13) was order her own meal in a restaurant because the waitress was a stranger.
The therapy really taught me how to help her, as well as how to counsel her teachers at school, and others who had to interact with her. It made all the difference. Hang in there!