Huh… yeah, weird. I can’t speak to PanAmbi3O because it’s an extension from… somewhere. I don’t have it installed and it’s not stated where to get it, so I can’t try it here.
But if Pan2 reproduces it, then a “minimal working example” would use Pan2 (minimal reproducer should prefer core UGens).
The first thing I would check, just to be sure, is:
sig = PinkNoise.ar(0.2);
sig = Pan2.ar(sig, 0);
a = Synth(\test);
s.meter; // here!
This is to confirm that both channels are being written to the L and R output busses. This should be the case, but, when you’re seeing weird behavior, it’s good to eliminate all possible variables.
If you see only one channel in the vu meter, then something is very weird (= this should be impossible).
If you see both, but hear only one, then the problem must be in the connection between scsynth and the audio output. This is much more likely.
Which OS are you running? Because the connection mechanism is different per OS.
If SCNvim, I might assume Linux. If so, what do you get from the following?
// useful in Linux; not useful in Win or Mac
system meaning that scsynth out_1 → system:playback_1 and so on, in sequence.
Are you changing anything in ServerOptions before booting the server?