why and what’s this?

It's always hard to say, a lot of people mentioning Ca, but is it rly? i had a same problem and gave my plant more and more Ca and feed and it was becoming a bad, at least not stopping how did you managed pls let us know

Calcium deficiency. Its coco, just have it always in small doses in your feeds. No other way.

Amigo a mi me parece más un pH desajustado, por el color irregular de las hojas Riega hasta que haya algo de escorrenteria (flush) y mide el pH del agua que sale por debajo de la maceta Debes tener un pH entre 5,5 y 6,2 Si no es así, riega con una solución con pH ajustado hasta escorrenteria para arreglarlo...

You need calmag with Coco . Especially if your in a soft water area. My tap water is 0.1 EC. So I will add magne cal plus up to 0.4 every feed. Then add you a and b on top of that. Coco requires a calmag buffer .

Fix your PH, Heat, and ir Circulation Problem

you didn't fix the calclium deficiency. Looks like a new spot is forming on that upper leaf. Mg deficiency takes 30+ days from inception of the deficiency to the time you can see a symptom. Your plant doesn't look that old, so the symtpoms you see are not Mg related. Mg deficiency is mutualyl exclusive from Ca... Mg deficiency will have spots accompanied by interveinal chlorosis, which i do not see here.. not Mg for 2 reasons, and feeling confident about that. Diagnosing this shit is always a "maybe" .. familiarity with what you do can give a lot of confidence, but i'm looking at your plant with incomplete info. So, i'd wager it's still a calcium issue. Calcium is not somethign you add once in a while. You or your soil should have enough at all times... if you needed to supplement it before, you'll need to supplement it until the end. if in soilles, it's fairly easy.. 100ppm or more in your fertigation and assumes you fertigate every time with 10% runoff or more, againif in soilless. Soil is more complicated. you got whatever is in the soil that slowly depletes and you may not fertilize every irrigation, so you'd need more inversely relative to frequency. less frequent = higher concentration... it'll average out roughly the same over time, though (law of conservation of mass playing a key role here)