Mono Line Input Turned Into Stereo By Adat Output

I have connected my MR816X to my UR824 by Adat, to use the UR as my main interface with the MR providing more inputs.
However, I have a mono synth going into input 1 on the MR, but it is showing up as a stereo signal on channel 1 and 2 on the UR after its journey down the light pipe.
I’m hoping its a simple setting I’ve missed somewhere, but I can’t find what I want in the manual, or searching the internet.
Any help greatly appreciated.