Hi @Miscreant
Are you dragging the midi onto the Scaler track or the Piano track? I suggest that you should drag the midi onto your Piano Instrument track, and do not route any midi from Scaler to the instrument track. To see how to route Scaler in Reaper you may want to look at Basic Introduction to Multi Out. Although this tutorial is about routing multi output from Scaler it does cover routing in Reaper to trigger instruments on instrument tracks from Reaper.
I suggest you may want to select OFF in Scaler Instrument selection: this will silence Scaler.
Hope this helps