okay thanks, well this context changes things and does raise some suspicions of the setup.
if these are both true:
then i have to suspect some kind of phase cancellation or other weirdness from the wiring. the crucial part is how the thumb piano TRS is wired.
in the first case, nothing is shorted - norns L/R are getting (S,T) and (S,R) from the source. if the thumb piano outputs a balanced TRS signal then this explains things because it’s wrong: in a balanced TRS cable, T and R are the same signal, but 180 deg out of phase. so when you put them into L and R norns inputs, and then sum those together in the box, they will cancel.
(whereas in the monitor path when monitor mode==stereo, you will hear the same signal in both ears, but in opposite phases - which is not however obvious to your ears and brain.)
in second case, well i’m not clear what OP-1 input really looks like but seems like you are shorting R+S at the source before anything else happens. (unless you meant TRS->OP1). if my theory above is correct then this will now work because you’ve effectively thrown out the negative-phase signal, then split the resulting mono signal in the OP1.
(again this is your thumb piano has a TRS mono balanced output, as opposed to TS mono output, or TRS stereo output.)
i would check the following:
also sorry if you said this, but is this issue totally repeatable or is it intermittent? if repeatable, have you tried other (e.g. line-level, mono) signal sources?
