MIDI only plugins (no audio in or out) hosted/patched in row in the AudioProcessorGraph have no MIDI out and can not alter MIDI.
How to reproduce:
Host two MIDI plugins (A & B) in the JUCE TestHost (B needs to be the MIDI only one).
Route the MIDI out of A in B and the MIDI output of B to the main output.
A generates MIDI notes and B clears all its MidiInputMessages: the output of B is the output of A (no messages removed).
A generates MIDI notes and B adds some more: the output of B is the output of A (no messages added).
B’s MIDI out is simply ignored.
If we add an audio input or output to plugin B everything works as expected and the output of B is a mix of A and B.
Tested with VST3 plugins on Windows.
But it seems a problem with the graph not with the plugin implementations.
Is this the right place for this report?
If you need a tester for the fix please let me know.