After finally getting my 2nd piece of tier I've noticed some examples of this behavior, probably stemming from fixes to keep Unerring Vision from getting out of control.
Normally, when Shadowpriests have a haste buff, we're able to take advantage of DoT snapshotting to extend the duration of the benefit, or at least stack haste buffs in order to gain extra ticks on our dots. However, the current implementation of the T15 2pc, negates not only our ability to do that, but the effect of temporary haste buffs on SWP and VT entirely.
Example of the behavior I'm seeing:
I cast SW:P with Berserking and my SW:P starts ticking on the target every 1.62 seconds for 17.82 secs.
A Shadow apparition spawns, and hits the target.
Even if berserking is still active... the SA will add 1.62 seconds to the duration of the DoT, however, the tick period is then reset to my non-berserked haste, 2.26 seconds.
So, if I had 12 seconds remaining on SW:P when the SA hit, the SA increases this to 13.62 seconds, but then the dot will tick 5 times every 2.26 seconds after the last tick completes, and then fall off the target with 1.66 secs remaining on the duration.
Normally, my berserked SWP would hit for 11 ticks (not counting mastery), but with the 2pc in this last example it would wind up hitting for 9 ticks, including the extra SA tick. So, I wind up losing 2 ticks overall compared to a set without the 2pc.
Even weirder still, occasionally, after the UI might say your dot has fallen off (due to a partial duration remaining), a SA will hit and effectively 'refresh' the invisible dot, although your UI will not show the DoT is still there, but you'll see the damage in floating combat text, and the combat log.
This effect occurs with both haste rating buffs (Volatile Talisman of the Shado-Pan Assault), and % based buffs (Berserking, PI, Legendary Meta).
Looking further into this behavior, I found a silver lining in that it looks like any dot extended by the 2pc will gain any active int buffs at the time of refresh. So, if you're not good with refreshing DoTs for int/spellpower procs, the 2pc seems to do it for you.
But this is a double edged sword, as it works well when an int proc happens to fall off while a dot is active. However, when the dot is extended it will won't replace any effects that weren't active when the dot was cast.