That sounds like half of your audio group's output channels aren't sounding. If you have, for example, two stereo audio outputs in the audio group, going to 4 speakers, half of the notes in each octave would go to one stereo pair, and the other half of the notes go to the other pair. That's assuming you kept the default rank routing algorithm, which splits each octave into c and c# sides chromatically but flips the sides for each octave, and chooses sides based on the rank number.
So if one stereo pair of audio interface channels, amplifier channels, or speakers is disconnected, turned off, muted, or broken, you'd get this effect.
If you have two mono outputs instead of stereo, it's the same idea.