Similar LED, higher voltage drop, less bright
If there is a higher voltage across that LED, than it is drawing less current because the voltage across the 220 ohm resistor is less (5- 2.1 or 2.9 volts compared to 5 - 1.9 or 3.1 volts across the other 220 ohm resistors.  Thus it is drawing 2.9/220 or 13.2 ma while the others are drawing 3.1/220 or 14.1 ma. Generally, LED brightness varies in step with the current through it. That being said, the difference between 14.1 and 13.2 is not much so that the brightness difference shouldn't be that much.  Do you have the specs on the LED?  It may be perfectly normal for that particular LED to have that variation of voltage drop. I don't believe you damaged the LED nor have any circuit errors.