WDI
I don't think it's a question of who's at fault. I think it's just the nature of bpm synced effects. I believe even hardware acts this way. For instance the Korg Electribe has bpm synced effects that sound horrible during tempo changes when synced to sonar.
So it's either the way Sonar sends tempo change data or just the nature of bpm synced effects. I'm guessing it's just the way bpm synced effects behave during tempo changes.
Actually, hosts do not send tempo changes to resident plug-ins; the plug-ins ask the host for this information every time they get the nod to do some processing. At least that was the way things worked prior to VST3, which I haven't looked at (although it's doubtful that aspect was changed).
Any plug-in that uses delay buffers has do deal with readjusting buffer content and position when tempo changes. Some are obviously going to do it more gracefully than others.
Anyway, considering VST 3 brings up a possible explanation as to "why it worked before". SONAR now does VST 3. Any chance the plug-in now being called by X3 is a VST 3 version and you were getting a VST 2.x version before?