I think I wrote a post about this some time ago....it's an annoying bug but not a disaster....don't think anything conclusions were arrived at then and to be honest I pretty much forgot about it. But tonight it happened again and I just wondered if anyone else had experienced it. 
 
Normal behavior: you select one clip in a track, then SHIFT-select another one. All clips between (and including) the two clips will be selected. 
 
For the second time, I now have a MIDI track which doesn't behave like this! It was working yesterday and all of a sudden it's not. What happens is: I SHIFT-select the two clips, and the relevant portion of time is highlighted on the time ruler, but the clips in between are not selected. They are dimmed, but not selected (i.e. if I then hit DELETE, only the first and last clips are deleted). 
 
Like I said it's annoying but no biggie...I can still lasso the clips in question and they're all selected. Just wondered if it was a known bug. 

In X2 most of my problems are around my perception of what is selected or in focus. Often times, I'll click on a given track and adjust a PC that is actually tied to some other track. Or I'll be looking at one thing and adjusting another. For example, the other day a master bus ended up down 6 DB... why?? when did that happen? I never had this problem before X2. I think that the model for focus has gotten more complicated and sometimes it leaves me behind.
 
Now I will say that when I'm paying close attention, there is always enough information to know what I'm modifying, but I misunderstood all the time when I started using X2 and I still mess it up after 7 months of X2.
 
Sharke, I wonder if you hit ctrl+shift+a (de-select all) before select the range if you can repeat the bug.
 
Let me describe a circumstance that would result in the confusion you are describing... you have focus and selected on track 1... You add focus to track 5 but do not move the selected value. You shift click track 2. Now, range of selected should be between track 1 and 2 by design, but you might think that it should be between track 2 and track 5.
 
Anyway, I'm not trying to say it isn't a bug in the software or even discussing bugness. I'm trying to help identify if the problem you are bumping into isn't one of understanding the new 'Selected' vs 'Focus' model.

I know what you're saying about focus confusion - believe me I've done my fair share of twiddling mix settings on the wrong track in the inspector - but this isn't affected by focus. Regardless of the track you're focused on, SHIFT-selecting a range of clips should always work, regardless of the track they're in. In this case, the problem is still evident when I deselect everything. The fact that it only affects one particular track suggests to me that it's a bug. It seems to be one of the many little oddities that "corrupt" my projects as they get older. I've actually abandoned a couple of projects because things got too weird inside them. 

This falls under the myriad of selection issues I've had with X2. Once they start infecting my projects they are there to stay. With this particular issue I just resort to keeping an eye on things and using Ctrl + Click to select the items (it happens in track view, PRV, wherever) that got missed.
 
:-(

I hear you Sharke. It was a wild guess. Hopefully they'll get the bug sorted. Maybe it's a variant of the quick groups + ctrl click on mute and solo where you are toggling the track so it is either muted or it is solo'd.
 
I don't know. Hope they fix it.

I'm currently in the process of eliminating every possibility of the causes of my sonar problems. I'm three whole days into it. I Want To Make sure I've given sonar every chance. I'm not finished yet but at the moment my findings indicate that individual projects become infected like you said and then there's no going back. That will be disastrous to productivity if true and can only result in me leaving sonar for another daw.