$500 is not Recovery Rebate Credit  - it is Other Dependent Credit   not the same thing and                yes - the parent's lose that credit if the child is no longer a dependent - such is life.

Yes, the child, now on his/her own will receive $1400 Recovery Rebate Credit even though the parent's received $1400 for this same child -   The EIP payments were done is based on 2020 returns with the possibility there would be overpayments and underpayments.  Those lucky enough to be overpaid do not have to pay it back. 

I've encountered this situation many times.... doesn't seem right but that is how it was designed to work,