I'm having strange behaviours enabling sidechain on a mono track in logic, midi or audio doesn't matter.
When choosing a bus from the sidechain menu I see the signal on the right channel main meter (I shouldnt..); even more strangely if I enable the SC high pass filter the complete sidechain signal is superimposed on original track's signal both in level meter and in audio playback (and it's not high passed).
On a stereo track it works as expected.
Can you reproduce this? I installed the last update but nothing changed.