That is normal, not a bug. When arming a track, all the plugins connected to the track (i.e. if you have some heavy effects on a send) are put on the same core since they need to be processed in series.
In the above case with 16 sends connected to a single track that's what you'll get, since all 16 reverbs need to be processed in the audio RT thread.
Try using less patches in a Kontakt instance connected to less reverbs. But I will also say that 16 reverbs is quite an overkill anyways.