Strange problem with imported MIDI files

So I’m working on a mix for a band and it’s a mixture of audio files and MIDI files. I’m experiencing a weird problem where MIDI tracks are kind of…linked together. So I solo the bass track, and the piano track is solo’d as well. Any idea what would be causing this? Even if I create a new track, move the MIDI part onto it and then solo that channel…other MIDI tracks are being solo’d along with it, almost as if the MIDI parts contained instructions to link the tracks together. Never had anything like this before. Pretty weird! Cubase 7.5.20 on Windows 8.1 64-bit.