I would say if it stinks like bad farts or bad eggs that's root rot and more so what with system your using, feeding from bottom is always gonna carry a possible risk of root rot , not that ive seen it happen in this system cus they're not really sat in your mix long enough, but its more dodgy bottom feeding by hand letting them sit in your mix too long.
But yes what with looking at your plants now they do tend to yellow up with root rot. I dont know how long is left on this but it might be an idea removing it from your system and feeding through top for a while with some silver bullet or h202 in your mix that you give it or at least adding add some silver bullet in to your current tank if your can`t be bothered with top feeding thing, just to use it as a preventative measure for rest of plants too as well, cus that rot can spread if any of it has got into rest of system although it would be pretty hard for that to happen in this system. You`ll need to clean the system out or at least the part used by this plant good and proper obviously.
So id prolly look at buying some silver bullet to use in that set up from now personally speaking and just used as a preventative, I've not used it but i read silver bullet is a better product that h202. That's if it is root rot, but it seems to point that way now what with how plant looks and your stinky roots as and also not being able to pin it down with what was up with plant. Air-stones or flooming your mix in your tank should help keep ferts oxygenated longer. Try to keep temps in your tank around 20c`ish higher temps less oxygen in your mix, silver bullet should help with this too