Multi Out changes note for MIDI channel 2 at random

Set up Multi Out with Reaper and everything seems to work just fine, however if I Solo Bass in BBC Core Spitfire and remove Solo playback the wrong note triggers and goes out of tune with the rest of the chord/instruments: Celli Channel 3, Violas Channel 4, Violins Channel 5.
If I Solo again some chords will play correctly and others will still play with the “Bass” Channel 2 midi note out of tune.
Is anybody else experiencing MIDI notes changing like this?
I imagine it has something to do with the Spitfire Library vs. what Scaler is triggering but it could just be an oversight with my routing.
Any help would be appreciated.