Really strange issue with Ubiquiti router and port forwarding

Here is our environment.
UAP-AC-Pro with current firmware.
Client has an NEC PBX with a SIP trunk.
I have setup port forwarding according to the NEC documentation but when checking the router externally for open ports one port forward shows as open and the other not.

The ports that show as open are the SIP ports and those that are closed are the RTP ports.

How the heck can one be working and one not when all the settings are the same except the actual port numbers? Destination is the same ip and everything else also identical except the port numbers.
SIP-ALG turned off as is UPnP.
Got me completely baffled.