Congrats on the no-problem grade!
I'd guess the defect is a lamination which separated from the coin. It would qualify as a mint error but probably wouldn't enhance the value much. I wonder if 20-cent pieces have a tendency towards lamination issues. My 1878 proof coin shows a lamination-type error at 2:00 on the reverse which has not separated. If the peeling metal segment did separate the resulting void might be similar to the affected area of your piece.
