Hey, how's it going?
I would guess it's the beginning of a slight phosphorus deficiency.
But that doesn't have to be the case. Phosphorus is best absorbed at a pH level between 6.5-7.5. This means there might not actually be a deficiency in phosphorus, but it just can't be absorbed well.
Maybe try adjusting the pH a bit. Next time you water, aim for 6.9 and then for the following times, go for 6.5-6.8.
When growing in soil, the optimal pH level is between 6.5-6.8.
Have fun and good luck with the harvest ππ