Port Forwarding - miultiple external ports to single internal port

I have the above and just tried to add a new Port Forward of port 12345 going to an internal address and port 987

I am getting told there is a conflict, so will do not allow it.

There is not, I do have a rule forwarding port 456 to the internal address 987, but that is all. Indeed, when I first rebuilt this router I have several forwards that take numerous external ports and point them to the same internal address and the same port.

what gives - how do I get around this bug in the OS ?

I do not fancy downgrading to an old version to apply this change and then upgrading again - as clearly it is wrong. and TBH, I would probably move over to DD-WRT again