maybe, but ...Observation, when output low there is a small current thru RED LED, eg. its log curve at low V,
and if modern high bright LED used it possibly can glow at a low level ?
If you swap the two LEDs, now the difference Vf difference between the single LED and the LED + diode string is significantly (in Vf terms) greater. The same is true if both LEDs are red, but less true for green. I completely understand how conditions and components could combine to have the off LED glow dimly, but, with low-cost "standard" LEDs and room lighting, I have not seen it.