Same plant, same genetics....but sometimes one just doesn't follow suit with the other siblings. If you look at your newest growth on that plant, it looks to be trying to grow through whatever the issue is/was. I wouldn't flush it...steady goes the course. Good luck.
This plant did have spider mites and was sprayed heavily when it was younger. Could that be the problem?They all have the same genetics. Likely the pot was contaminated. Or something fell in during transplant. It would be a good idea to flush it. Run 5x the pot size in water through it or under a tap for 10 min ( that is a small pot so maybe 5) and it should heal up.
There is no reason a clone from the same mother in the same soil mix would be drastically different. There are many ways it could have been contaminated including the cutting itself. But more likely is the pot had some residual stuff in it. Or it is in a dirty drip pan... something like that most likely.
That said it doesn’t look to be the same genetics of the plants around it. Could it be mixed up somehow from different mother?