it's called 'senescence'
https://en.
wikipedia.org/wiki/Plant_senescence
https://en.
wikipedia.org/wiki/Plant_senescence
there's nothing you can do about it it's normal...see all the normal healthy leaves, they all have proper turgidity (osmotic pressure), and no purple stems? taht means everything is going along normally for the plant...they just decide to kill off a leaf here and there every once in a while...nobody knows what causes it. Do not change feedings or waters when the majority of the plant is healthy and growing well. You'd just be chasing something that cant be fixed and mess up what was perfectly fine. ur fine dude dont sweat those few leaves.