It’s quite amusing that the reason I didn’t come up with the correct result myself is that I didn’t realize when I had turned on force mono…
What made me confused was the relationship between input/output and insert. The user manual is also a bit cryptic about this.
In fact, in insert mode, I/O 3 and I/O 4 are equivalent to Insert 1/2, and I/O 12 represents the left and right ends of the routing.