This is a great point, I think I might have been wrong actually. It doesn't really make sense that one row of the embedding matrix is treated differently than another...
Token Features 0 => list[1, 5, 6, 10]
Token Features 1 => list[7, 8]
...
These "list features" would be invariant to Caesar cipher. So then the LLM could pass these list features to a learned Caesar cipher decoder unit to spit out the decoded text.
It's still unexplained, however, why the Byzantine Music Notation would trigger the this circuit while other Caesar cipher's wouldn't.