damn, she's having some trouble, sorry to see that. The leaves above and below the bad ones look ok. That's kind of interesting.
You're running 24 hr light on? Reason I ask is you said it's cold so you heat with the lights. In the picture its 69F, is that under the lights or did you take it out of the tent to photograph it?
To me that seems a little cold for lights on. Not real bad, just a little. Unless autos like it colder, I searched but don't know. If they're sick in the roots too hot or too cold temps would make it worse.
You didn't say if you added perlite but I searched on adding it to FF and there are differing opinions but it leaned towards adding as much as 30% perlite. (
https://www.rollitup.org/t/should-i-add-perlite-to-fox-farm-soil.155270/) So re-potting would be a desperate measure on an auto but I trust ChefKimbo sees a roots problem. I think perlite is going to help get oxygen to roots to help them recover. Whatever you do I hope they get better for you. How are the other 3 doing?