Is this some kind of deficiency or a genetics-thing?

I agreed with @ORGANOMAN : get it out of this tiny pot ! But maybe not directly to a 15L one (especially with AirPot, use a 3L first).

The problem is that it is in a tiny pot. Try transplanting it into a container of at least 15 litres, with fresh soil. A plant that size in a container of the size it is now it will have barely any nutrients left, even though you are feeding it, but due to the extremely limited space for root growth, your plant can not access them, nor can it grow under those restrictions. If it doesn't recover after being transplanted, I will eat my hat! Hope this helps,... Organoman.

In my honest opinion, this is a PH issue. You may need to adjust your input based on what your runoff reads at. I hope this helps!

not super experienced here but check out growweedeasy.com it has a bit for sick plant symptoms and iv used it a few times now also has the remedies to fix them. bit of a weird angle on the pic but at roughly that point i had repot mine and they had been under a t5 veg light at 60w from start so maby check the site out and try tweaking a few things. i had a few issues at the start with temps and nutes but mine a pretty good now so dont give up dude. good luck!!