First, double-check the synth settings. If any (or all) of the synth's voices are set to omni, that could account for the strange behavior. Each voice would play correctly when soloed but interfere with one another when played concurrently.
If that part checks out, then we have to assume that the synth is receiving unexpected data. Go to the List View and look at the early events there. I know, you've already done this, but you may have missed something. Problem events don't always stand out at first glance. Make sure you enable all classes of events, including RPNs, NRPNs and SysEx. Try experimentally deleting all of the first measure's events and see if the symptoms change. If it does, then you know you have to zero in on that portion of the event list, perhaps deleting individual events until you identify the one that was causing you grief.
Is this project an original composition or a MIDI file you obtained elsewhere?
bitflipper's post in [SOLVED] MIDI Outboard Synth Issue was marked as the answer