It's not a nutrient issue, I tested the pH and EC.
I also doubt it's dehydration or suffocation, because it's in an optimal hydroponic environment.
So, there's only three options left I guess.
Option 1: wind damage.
It was quite stormy about one or two weeks ago. Maybe it got root damage (unlikely, small plant), or it bend too much?
Option 2: temperature fluctuations.
At the same time of the storm, the night temperatures dropped quite a bit (lowest: 12°C).
And now, it raised to 40°C just a few days after.
Or,
Option 3: sunburn.
It's under a shaded cloth, and gets direct sun only at afternoon, if at all.
All other plants (e.g. pepino) do very well and have no light stress.
Or is it something different?
It also lost a few pollinated buds. Is that a very bad sign?
New growth this week has appeared wrinkled as well.
That's quite a temperature swing. I'd also note that Capsicum Chinense family likes more dry conditions, whereas the Annuum (Thai) are used to more humid and wet environments. Probably why that one looks healthier.
General rule of thumb: thin skin peppers like dry, and thicker skinner peppers like more water. If you try to grow Bell peppers next to Aji peppers in the exact same soil conditions, one will over overwatered, and one will be underwatered, if that helps illustrate the difference.
Thanks!