• Arduino by default assumes traditional serial MIDI
• Adafruit TinyUSB Library for Arduino supports MIDI,
  but neither for STM32 nor composite USB devices

PlatformIO USBLibrary for STM32 - ported to Arduino
  implements Arduino PluggableUSB API, supporting:

• FortySevenEffects / Arduino MIDI Library
  can read all MIDI channels, supports callbacks
    and multiple MIDI interfaces, for mergers/splitters applications.
• depends on lathoub / Arduino-USBMIDI, which in turn
• depends on Arduino MIDIUSB, based on PluggableUSB.
• MIDIUSB2 adds callback support to MIDIUSB
• MIDIUSB Arduino Library doc