yes the “I’ll contact you again between my 18th - 21st birthdays.” Sounds very weird and unsettling. That’s a strange thing to write to your family.
It sounds believable to me—actually, it resonates with me. When I was 13/14, I’d made specific plans for something I was going to do when I reached 18+. (It turned out that I didn’t need to.)
Since she was taken away from her parents, could she even have been trying to protect them, by not coming back to visit until the state doesn’t have control over her?