Rolf: Again, this is incredibly useful information. This leads me to believe some Teensy boards aren’t supplying enough power to the IR receiver. I would be curious to know if using the “good” Teensy board on the troublemaker fixes the problem there. Likewise, the only other thing that uses 3.3V is the microSD card, so a final test could be to swap those and see if there’s any effect.
If you have time to test these two things sometime I would (again!) greatly appreciate the help! In the meantime, it’s good to know the Photon solved your problem.