Thanks for the input.Hard to say, if I had to guess I would go with overwatering.
These WW when overwatered tend to show signs of deficiency similar to those of cal-mag deficiency with burnt dots on the leaves... I think it's unlikely that this happened with the one in the red pot, but it's in quarantine for that very reason, I'm not sure what's happening. Usually water twice a week, considering 25°C, has been evaporating fast. Yesterday I inoculated all the plants with AACT made from compost, mycorrhizae and a pinch of Kelp Meal. Let's see how it reacts.
But I'll take it into consideration and let it dry well before watering it next time and keep it that way for a while.