There is no such thing as an actual mono, as in single channel, track in REAPER. All tracks have (at least) two outputs.
If you click the in/out button at the top right of the plugin window, you can see the outputs of the plugin. You probably have only channel 1 output checked on one or more of your plugins. That would create the behaviour you are hearing, as the default pan is actually a balance knob, so turning it 100% right mutes the left channel.
Here is the correct routing for getting mono output: