On Reverb Bus

The "pre" setting means "pre fader", not "pre send". So that means the send is not influenced by the level of the track. So in this case, soloing the piano has silenced the vocal track, but not the signal being sent from it. If you set it to "post fader", that will silence the send when the track is silenced. However, the send level will now be linked to the level of the track.
 
This is actually a more typical scenario, as the level of your reverb send will maintain the same relationship with the dry signal when you vary the dry signal's level.