I popped open an X3 project which had one song per track to start playing with things in Platinum. Initially I did not realize the solo'd track was looped, and found an odd issue. Here is what I did:

1. This cwp has one song per track, so I globally unmuted then remuted the entire project; then I unmuted/solod the track I wanted.
2. Rewound the transport, and began playback.
3. There was an existing loop enabled from bar 2 - bar 8 which I did not catch on the first pass.
4. Second pass on the loop I disabled the loop about 2 bars shy of the end with the transport running (this short distance seems important).
5. When the transport reached the end of the loop, the transport kept going, but all audio went out (like the track muted itself).
6. Re-Enabled the loop, rewound the transport, and attempted to replicate. Audio engine crashes on the second pass.
7. Re-Enable Audio Engine and try again... this time things are fine.

 
This recipe has been consistent for 3 tries, and is basically audio mutes, then the audio engine drops, then is fine.
 
Edit: This recipe resets itself by simply closing/reopening the project.

I'm guessing that's a disk buffering issue where SONAR buffered audio for the beginning of the loop and you tricked it by disabling the loop at the last second, you sneaky bastid.   Probably exacerbated by having to buffer up a lot of stereo audio tracks that are muted but still have to be loaded in case you unmute one.
 
Try increasing playback disk buffer size in preferences and/or archiving some of the muted tracks, and see if it goes away.
 
Not a Platinum-specific thing in any case. I've run into that in earlier versions.

Yeah, the timing of the unlooping seems to be critical in getting this "drop out" to happen. Perhaps I am just grabbing at straws to "crash something!" In the grand scheme of things it doesn't cause any issues... The most I could do was get the audio engine to drop :(