1. Do you use the latest Teensy dev/platform for PlatformIO? Please run `$ pio update` in IDE Terminal
2. Could you try USB_MIDI_SERIAL + Faster Optimization profile in Arduino IDE. Does it work for you?
Hm... We've reproduced this issue now. The problem is linked only with Teensy LC. The only for this board a default optimization profile is "Smallest code". We introduced optinization profiles a few weeks ago. You can...
Ok, that was the wrong message by me before. Actually, it works in the same way as with Arduino IDE. Could you provide non-working PlatformIO project? I used the next configuration:
platform = teensy
board =...
Do you use the same optimization profiles? USB settings? See docs how to configure them https://docs.platformio.org/en/latest/platforms/teensy.html#configuration
Can I reproduce this issue without MIDI device? I have...