Here is another pic. I’m using FFOF and 20 days in. Just been water except this last water on Sunday I gave them some recharge.
It could be just the strain. I see you have stem and branches exposed with your lst, the direct light can sometimes cause red/purple and more harden, nothing to be concerned with if that's it. When I get stiff limbs I'll gently roll between my fingers back and forth to soften then gently bend. If it's not those it could be to large of temp fluctuations, check how cold it gets. Also check your ph frequently, to far of fluctuations can cause similar issues in plants. Seeing your leaves look good and show no signs id just check things as precaution, but not make any changes yet, as I started with it could be no issue at all.
@mwhite9829 i don’t think it’s the temp. With lights off it’s about 72 and 80 with the lights on.