Purple leaf stalk

I’m wondering if anyone could help me with this one?

I’ve just noticed that about 6 leaf stalks (all above 6 nodes) have started to turn purple like colour. Only the top half of the stalk (light facing side) has change and the underneath remains green and unchanged
It’s slightly dotted in consistency

Could it be possibly heat damage or strain
Laughing Buddha sativa 80/20
Canna coco + perlite - 5 gallon pot
Canna a+b 3/7 ratio around 950ppm
Day 55ish
10 days into flower 12/12
Lights are t5x 4 so they do get quite hot and are around an inch away from top of the plant

I'm going to say it may be the beginning of phosphorus deficiency, not sure but its sounding that way, Some causes may be. over watering, soil has no air left this will affect uptake of phosphorus, low-temperature drops in the grow room of you have one to 55- 60 degrees Fahrenheit or 12- 15 degrees Celsius, high PH level above 7 or low PH levels below 6 causing a lockout, get a high PK fertilizer if it's none of the above if any of the above fit just correct it.

Plant looks great. As redeyedfrog posted, it could be one of a dozen things.
I stopped worrying about purple stems/stalks a long time ago.
If the plant looks healthy and buds are forming like yours are, don't chase your tail over it.

I agree with the above post

Your girl looks nice; don't sweat it. Carry on.

Sometimes it's the strain but as stated above could be phosphorus or magnesium or lots of other things. I don't sweat them anymore either.

Just check those 3 things I mentioned

Appreciate the replies all!
Ill alter the pk to slightly higher in the next nutrients batch I make and see what happens.

Cheers