Pan not completely panning

I’m using Audacity 2.1.1 on Windows 7. I’m trying to take a cast recording of a musical and pan it completely left and mix with the accompaniment track. The issue is that I want the chorus voices only on my monitors which I was patching in to the left mix out on my board and the right mix to be instrumental only, which I would patch to my mono speaker in the theatre house. When I pan (or set “left/right track”) it still comes through the other speaker, even though the Monitor bar shows only left or right respectively.

I thought I might have had a cheap sound card issue, so I moved over to my Mac running 10.11.6 and Audacity 2.1.3, which makes me think its a software issue. Any help on solving this issue? Is it user error instead?

Some sound-cards can apply a pseudo surround-sound effect.
That effect introduces crossover : some Left on Right, and vice-versa.
If that is the problem the effect can be switched off in the sound-card settings , (not part of Audacity)