The following test uses TTS-1, but any other multi-out soft synth (Kontakt, M1, etc.) can also reproduce the issue.
 
1. Load the Normal Project template, delete the existing two tracks, and then drag TTS-1 from the Browser (with "MIDI Source" and "All Synth Audio Output: Stereo" selected) and configure its tracks as follows:
 
MIDI Track 1 (MIDI Ch1) ---> TTS-1 Ch1 --->Audio Track 1
MIDI Track 2 (MIDI Ch2) ---> TTS-1 Ch2 ---> Audio Track 2
MIDI Track 3 (MIDI Ch3) ---> TTS-1 Ch3 ---> Audio Track 3
MIDI Track 4 (MIDI Ch4) ---> TTS-1 Ch4 ---> Audio Track 4
 
Note: To do this, you need to create three MIDI tracks yourself (which are MIDI Track 2-4) and select the appropriate channel from the MIDI channel drop-down and TTS-1 from the Output drop-down. Then, on the TTS-1's GUI, click [System] -> [Option] and select the appropriate outputs, like this:
 

 
2. In the Track View, group the Solo buttons of the first MIDI track and the first audio track. Do the same with the remaining pairs of MIDI/audio tracks accordingly, like this:
 

 
3. Click the Solo button of the first MIDI track. This automatically turns on the Solo button of the first audio track. Make sure the button grouping of other tracks works properly as well.
 
4. Turn off all Solo buttons, select all the four MIDI Tracks and then hit ALT+3 to open the Piano Roll View. Don’t maximize the PRV because we need to see the track strips in the Track View at the same time.
 
5. In the PRV's Track Pane, click any of the Solo buttons of the four MIDI Tracks.
 

[Expected Results]
It only turns on the Solo button of the paired audio track automatically.
 
[Actual Results]
It turns on the Solo buttons of ALL eight tracks (you can see this in the Track View).
 

Reported on July 13, 2013
Confirmed and submitted to development on Oct 19, 2013

A lot of steps merged into step 1 so it's hard to reference.
I have set the soft synth itself for four outputs as instructed.
The four tracks that were automatically created when adding the soft synth has Output 1/Stereo, Output 2/Stereo, Output 3/Stereo, Output 4/Stereo.
 
After I created an extra three MIDI tracks (4 total) the only MIDI output I can set each track to is Cakewalk TTS-11 ??
 
Thanks...

Yeah, the steps were originally created for CW but since the Step 1 wasn't clear enough for those who aren't familiar with multi-out synths I ended up cramming additional info into the step when I created the post. Sorry about that.
 
Below is a simplified version of the steps to reproduce the issue (you don't even need to toggle TTS-1's GUI):
 
 
1. Load the Normal Project template. Delete the existing two tracks.
 
2. Drag TTS-1 from the Browser (with "MIDI Source" and "All Synth Audio Output: Stereo" selected) into the Track View Clips pane (this creates four audio tracks and one MIDI track).
 
3. Select the MIDI track and hit Shift+Num0 three times (this creates three MIDI tracks).
 
4. Select "Cakewalk TTS-1 1" from the Output drop-down of all the three MIDI tracks.
 
5. In the Track View, group the Solo buttons of the first MIDI track and the first audio track, by right-clicking on each Solo button and choosing a desired color of groups. Do the same with the remaining pairs of MIDI/audio tracks accordingly, like this:
 

 
6. Select all the four MIDI Tracks and then hit ALT+3 to open the Piano Roll View. Don’t maximize the PRV because we need to see the track strips in the Track View at the same time.
 
7. In the PRV's Track Pane, click any of the Solo buttons of the four MIDI Tracks.
 
 
[Expected Results]
It only turns on the Solo buttons of one audio track and one MIDI track (paired tracks).

[Actual Results]
It turns on the Solo buttons of ALL eight tracks (you can see this in the Track View).
 
 
The reason why I want this bug fixed is because I want to group the Solo buttons of a drum map track and its source MIDI track and be able to solo the drum track with a single click in the PRV Track Pane (actually a single press of a key because I use a shortcut key for soloing the current track).

Yup totally reproduced. Thanks Jlien. Good catch.

According to Jlien X not fixed in X3E.

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