Ill try to use the INPUT_PULLUP of the uno to connect it otherwise mayby this will solve my problem.
Can it be that the cabel is making too much resistance that the pullup resister is not seen anymore as a damping but as a divider resistor, the cables are atleast 2 meters long so the travel distance is 4 meters
After you find out what's blowing up the Unos, you still have the issue that the Teensy 3.6 I/O is not 5V tolerant. A TXB0104 might be simple solution although I really want ask... what is the Teensy doing? I don't see any I/O connected. That's a lot of computing power for a serial connection.