@sno @tehn this sounds like a symptom of MLR engine smashing the main input busses on engine shutdown. this should be fixed with the recent update.
if this theory is correct, besides going away with the update, the problem should consistently appear after switching from MLR to a different script, and then back to MLR.
if its wrong, then maybe there is a similar error remaining that i didn’t spot, or my fix was bad.
it’s an engine limitation, but it’s not impossible to extend the engine/ugen. since the ugen is still undergoing many changes, it doesn’t seem like the best time to extend it to multichannel - this involves a significant amount of c++ boilerplate and is best done when things are a little more settled.
besides the lowest level DSP, there is of course some usability stuff to think about to switch transparently between stereo/mono modes.