The problem isn't specific to X2, I have seen the same thing with both 8.5 and X1.
It doesn't necessarily happen every time, which is why some users say they haven't seen the problem. It seems to happen on certain large and/or layered samples (meaning most patches in Omnisphere), and may also depend on drive speed or disk buffer sizes - I haven't figured out the exact combination so I just make a habit of slow freezing/bouncing/exporting Omnisphere tracks.
In fact, I usually turn off the fast-bounce option when exporting in general because Omnisphere isn't the only plugin that doesn't like fast bounces. Rather than keeping track of them all I just always do a slow export, which avoids several potential problems.
I did, however, once have a project that would screw up with fast bounce but also screwed up (in a different manner) with a slow bounce! That project had both SampleTank and Omnisphere. SampleTank would ignore some MIDI automation when slow-bounced, so I had to freeze it first and then export slow for Omnisphere's sake.
Hey, if this stuff was easy
anybody could do it!