I can reproduce this,
I load the Strings all VelXF patch and choose one of the Short or Pizzicato articulations with the Velocity Crossfade set to Off.The drop outs do not always come directly, somthimes I need to play around for a while before they occur. It doesn't matter if I play fast or very slow.
The drop out is random. If I just repeat the same note, over and over again, some of them will sound and som of them will not sound.
I use two mic positions, Main and Surround, sent to different outputs and then routed to different speakers. The drop outs will be very clear because of the different speaker pairs and in my case it's always the Surround samples that drops. The Main samples never drops for me.
My scenario exactly, except I also use the mid mics so the drop outs are even more obvious