If it isn't affecting growth, why worry about it?
Sometimes its genetics, sometimes temps, sometimes sunburn, other times potassium issues, but hardly ever a REAL issue.
Your plant will be fine and grow nice flowers all the same, red stems or not.
Most cannabis gets reddish leaf stems in my experience and is more noticeable when they don't!
I am sure your plant does not need "fixing", however it will need around 6 hours of direct sunlight to flower to its best ability.
Hope this helps,.......... Organoman.