ASA Hairpin error "Unable to reserve ports"

I'm having trouble creating a hairpin NAT on an ASA running 9.12(2).

First of all I have this configured for internet access.

object network INSIDE-SUBNET

nat (INSIDE,OUTSIDE) dynamic interface

Secondly I have NAT statements like this for a few internet accessible services on different public IPs.

object network OBJ-FTP-SERVER

nat (INSIDE,OUTSIDE) static 123.123.123.123 service tcp ftp ftp

access-list OUTSIDE-IN extended permit tcp any object FTP eq ftp

But when I try to add a hairpin like this I get an error saying "Unable to reserve ports"

nat (inside,inside) source dynamic INSIDE-SUBNET interface destination static OBJ-123.123.123.123 OBJ-FTP-SERVER service OBJ-SERVICE-21 OBJ-SERVICE-21

The service object looks like this.

object service OBJ-SERVICE-21

service tcp destination eq ftp

Does anyone know what I'm missing here? This very configuration has worked fine for the last 2-3 years.