If you're mixing in OBS, I'd say it's probably
Advanced Audio Properties ->
Audio Monitoring. If you need to change that setting on the fly, the
Advanced Scene Switcher plugin can do that for you. (it's matured into almost a programming language, in that any trigger or combination of triggers can cause any action or series of actions)
If you're mixing outside of OBS, and then passing that through unchanged, then check the settings in that external mixer. If it's an analog board, then you might be getting some analog crosstalk. In that case, try having some unused channel strips in between...but if you end up with a different board anyway in order to do that, then the different board by itself might solve it.