Hi. Could this be a cal-mag problem? It has alread...

No big problem. Water spots/burns, some are still on the right one in the back - at least that's what i suggest. But most of the tissue is intact.

older leaf near bottom, possibly close to medium? I'd wait for this to progress before reacting. Old leaves aren't as well-maintained by the plant and if it's close to an often moist medium, it can impact transpiration of that leaf -- i.e. local issue. I wouldn't remove it.. just let it go for now. Also.. there are calcium related problems and then there are magnesium related problems. there are no cal-mag problems. They can coincide with each other, but that's not how we label this stuff. They are to unique atoms with different visible symptoms. Mg deficiency takes 4-6 weeks from the actual physical deficiency to see leaf symptoms. If they coincide with calcium problems, they didn't even start at the same time, lol. Get a leaf symptom chart. google image search can help. Ask all the questions you want, but start looking at it and comparing.. seeing the deductive rasoning to help diagnose... realize that leaf symptoms are not discrete and you need more info than that to confidently diagnose a problem. This 'could' be the start of some ca deficiency, but i'd be patient and see how it progresses, if it progresses. Where it happens on the plant and how it progresses helps diagnose, too. Just because a nutrient formula works for some period of time does not mean it works long-term... a slow buildup or deficiency might takes months to cause a symptom. You learn a lot about a formula the longer you use it. if symptoms arise while controlling all other factors (something you can pretty much do with soilless/hydro context), that means the formula is out of balance. doesn't matter if it took 1 week or 2 months to cause a visible symptom. Though how long it takes is relevant to the scale of reaction.

Magnesium deficiency. One dose will not fix it, you will need to give generous amounts until the issue goes away, then give normal doses often, to maintain the balance. Remember too, what is showing in the leaves now, is a direct reflection of what was happening in the root zone 2-3 weeks ago.........so any evidence of being "fixed" will take at least 2 weeks to show up in the leaves............it won't happen in a few days.

https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg If you already adding CaMg it is likey not it unless something is blocking the update. Best to start a diary and link it to the question. this will allow other to explore your progress and making a more accurate theory on why its doing what its doing. CaMg can also react easily with a lot of the other feeds if not mixed in correctly. If your solution is turning cloudy at any point. it might be the issue.

Not enough information to say what the uptake problem is. Could be soil temperature, or many other things. Leaf temperature readings.

Feel free to give her some CalMag as a precaution. Personally, I add CalMag every watering so the issue doesn’t even come up and you can rule it out right away. I usually go with about 1–1.5 ml per liter. Happy growing!