Q: certain guitar plugins play in monophonic

Working fine for me on El Capitan.

Hi

Check your keyboard controller MIDI Transmit channel. These patches have "articulation/string" selection via MDI channel.

Set your keyboard to ch 1 and all will be well

CCT

The answer is that the Amp plug in turns it into a Mono signal.

Guitar amps, by definition, are mono devices.

1) "Classic Clean" has an Amp as the last plug-in in the channel, thus turning it mono.

2) The acoustic guitars do not have an Amp in the channel, they are Stereo.

3) Other guitars (e.g. "British Stack Synth Lead") have an Amp, but some Stereo-creating plug-ins after the Amp (e.g. Stereo Delay), thus turning the post-amp Mono signal again into a Stereo signal.