Show Cue System

We have a pair of speakers on stage, L/R.
I have these assigned to devices in SCS...
M-Audio output 5 Mono Stage Left.
M-Audio output 6 Mono Stage Right.
M-Audio Stereo output 5/6 Stage.
If I play cue to Stage Left, all is well.
If I play a cue Stage Right, all is well.
BUT if I play a cue to Stage (stereo) then further cues set to play on S/Right or S/Left ONLY play on both channels.
I've tried this in SCS 11.7 and 11.6.2 with the same result.
It seems as though once I play a stereo file to the stereo device, the individual L/R channels cannot be played to in Mono.
I realise that I could use just the stereo pair and then pan hard left or right - but that isn't the same when playing a stereo file to just one speaker.
I thought this might be to do with the way M-Audio works - but it's exactly the same with Dante Virtual Soundcard.
Any thoughts?

Are you using ASIO or DirectSound?

I'm using ASIO.

I just set up a new cue file with 3 cues.
Also created 3 audio devices o/p 1 stage left, o/p 2 stage right. output 3 stereo L+R
A plays stage left device (mono)
B plays stage right device(mono)
C plays to stereo device Stage (L+R)
I can play these cues, one after the other in whatever order, and they play CORRECTLY.
But if I close SCS and then restart SCS with this cue file, every one of the 3 cues plays to Both Stage Left and Right.

With that cue file open, could you create and email me a diagnostic file? You can create the file by selecting 'Create Diagnostic File' from the drop-down menu displayed when you click the Help button in the main window. The file will be saved in a folder named "SCS Diagnostics" under "Documents" or "My Documents".