I’ve got a audiograph containing AudioSources (adapted to AudioProcessors) feeding into external effects chains as well as midi channels driving VST instruments that can also be fed into external effects chains. The audio coming out of multiple AudioSources effect chains can be directly attached to an audio output node without mixing, but if a VST instrument is attached, a horrible noise, apparently a positive feedback artifact, is produced. The solution is to add a mixer to sum all the sources, but why is necessary to mix a VST instrument, but audio coming purely through VST effects can be connected without mixing?
Note the behavior can be reproduced in Juce’s demo plug in host with 3rd party VST instruments: just add two VST instruments and connect their outputs to the audio output. You must keep the stereo connections, I was able to attach just two lefts or two rights, but two lefts plus two rights cause the noise mentioned in the first paragraph.