To me there is no sign showing the signal gets in.
You talked about the Modular V3 in your first answer,
so I tried some experiments.
Here are my results:
Same as usual, I send an amplified guitar sound from the guitar track to the synth track.
Only the synth send to the master, so what we hear comes from the synth track.
If nothing is patched on the V3, we can only hear the amplified guitar sound, not modulated.
If I patch the ext plug to some in plug, the ext led lights up, we can hear the same guitar sound, not modulated.
If I patch more, enough to get a modulated sound out, It's the same. the ext led lights up, we can hear the same guitar sound, not modulated.
It's almost the same as my ARP issue, but the ext led lights up, strangely enough showing sign the signal gets in.
Weird as well I think, this led is very sensitive. If I mute the guitar track, it turns off. But it lights up starting at -140db, infinitesimal signal.