AK 2.0 XL Auto - Ganja Farmer Seeds

It happens. Autoflower plants are not entirely stable. Same thing happened to my wife plant. growdiaries.com/diaries/147240-grow-journal-by-the-hempster

Close up pictures of the EXACT issue would make this question infinitely easier to offer suggestions/help for. Please. Thanks.

That is not a nute toxicity of any kind... If you've got mushy leaves/stems, I think you've got a problem with root rot. Have you had problems with fungus gnats? They normally don't go after roots, the prefer to eat the decaying matter in the soil but when they run out of that, they'll start attacking the roots. Since it's an auto, I really DON'T want to suggest you unpot her to take a look at the roots (much too stressful!) - but she sounds like she's not salvagable anyway... I would get her out of the tent and take a look at the roots - there really isn't, as far as I know, any other reason this would be happening to your plant. Good luck...