in theory the kernel is the same between OG norns and shield.

Where is this sketch? Do you mean the midi_keypad.ino discussed on the Adafruit guide?

if I’m reading the library right, it’s only sending data if there’s something pending to be sent - not constantly flooding info every 10ms

void Adafruit_NeoTrellisM4::sendMIDI(void) {
  if (_midi_usb && _pending_midi) {
    MidiUSB.flush();
    _pending_midi = false;
  }
}

_pending_midi is only set to true when an event happens (noteOn, etc)


Additional thought - have you updated all the Arduino libraries for the Neotrellis M4?

FWIW - from your original dmesg output the only thing that looks odd to me is the [ 175.842650] sound midiC1D0: rawmidi drain error (avail = 431, buffer_size = 4096) error.