Hello fellow grower, Fox farm soil can be a little potent for seedlings even after rinsing and drying for a little bit.
Just judging by the pictures It does look like the young seedlings roots are getting slightly burned from the potent mix. Your other babys look like they are eating it up just fine this baby in particular might be a little bit sensitive.
I would just keep watering with clean water at a pH of 6.5 and let's hope this baby pulls through!