Tracks stop playing when entering key editor of a track

I have a piano track and a violin-track (All MIDI) in my project. On playback, when double-clicking the MIDI-part I’ve recorded and entering the key editor to edit the notes of the string-part, the piano stops playing. I tried adding another track, a cello, which also automatically mutes when I’m entering the key editor. The violin track (the track open in key editor) is the only one playing. When I leave the key editor, all tracks resume playback. I’ve tried open the key editor for the piano track, and also the cello, and the same thing happens. When I enter the key editor, the only track that’s playing is the one that’s open in the key editor. I’ve restarted Cubase and also restarted the computer, which didn’t solve the problem. I also checked other projects, and it’s the same thing. This suddenly happened, is it a bug? Or have I just checked a box or something?