D'oh! The elevator numbers are switched in the script for that level!
The elevator power box on the 1st floor turns the 3-4 elevator on/off (but still toggles the power line to the 1-3 elevator), and the elevator power box on the 4th floor turns the 1-3 elevator on/off (but toggles the power line to the 3-4 elevator)!
I think I accidentally removed and then replaced one of the elevators right before releasing the test version, which made them switch numbers in the code. Nice catch!