I use my condenser microphone with XLR input through a Steinberg UR-22 audio interface. Pro-audio equipment tends not to automatically turn a mono input into stereo. It relies on software having the option (or automatically) forcing any mono mic input into a stereo signal.
If I activate 'record microphone' in Plays.TV I will only get left or right channel depending on which of my two available XLR ports I use. This is not an issue I have with any voice comms software like Mumble, Teamspeak or when using OBS as they either mitigate this problem automatically or have enough advanced options for me to get round it.
I have the same issue with my RME audio interface.
I worked around it by using a software audio mixer. Plays.tv will accept a virtual mixer output as its microphone input.
I'm having this issue too, with my xlr microphone through a Scarlett Solo interface