When certain Audio Unit plug-ins are inserted in an Aux, audio from other Aux channels can leak into the signal path. In the image below, Aux 2 shouldn't show any output, but it's receiving audio that Aux 1 is getting from Bus 1.
This does not occur with the built-in plug-ins in Logic, nor does it occur with all Audio Units.
As a workaround, assign a Logic "Gain" plug-in with default settings (so as not to affect the sound) in the first insert of the Aux channel. You may then use any Audio Unit plug-in in the following slots without audio leakage.
This document will be updated as more information becomes available.