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.
Zoom in on your picture around the problem leaves. It looks like some form of pest on it. I have no idea what.
I looked at this and it's interesting: https://veggiepatchideas.co.uk/common-pests-for-chilies-a-growers-guide-to-protection/
Under thrips: "Grey to black markings on fruits are another sign. These markings can make fruits grow in strange shapes. This damage can hurt the yield of chili crops a lot."
Some pictures look like your problem leaves. Worth checking.
No pests visible, neither on old, nor on new growth shoots
Fair and good to check.
Is the black a mould or damage? Are you watering when sun is not hottest? (It doesn't look like scorching). How frequently do you water? Is it over watered? Are the chilli OK or not produced yet?
Damage
It's in hydro, as mentioned in the post. It's a recirculating drip system with LECA. The clay balls are constantly damp, and there's an overabundance of water, nutrients and oxygen at the same time. Zero root rot ever.
No harvest or fruits yet. The flower pods fall off. I've checked the EC and pH, and flushed the reservoir. There could have been slight nutrient uptake issues in the last days or weeks, but nothing major.