Mild deficiency — does anyone have an idea what they're lacking?

Magnesium is what's presenting, clear interveinal chlorosis. I don't see any nitrogen shortage whatsoever, nitrogen would start from the tip of leaves and work in on bottom growth if it was nitrogen. Doesn't necessarily mean its magnesium deficiency though it could be antagonistic (lockout) from I excess of calcium or K. The red or purplish stems are almost always a distress signal, and at their root, they usually trace back to how the plant's roots are breathing. People will yell P deficiency, or N this or that, but once you run the symptoms to root cause not just visible symptoms it all stems from a lack of oxygen. Whether it be soil compaction, poor porosity, high RH, over watering, take your pick. Soon as oxygen goes every plant process grinds to a halt, glucose conversion goes from 40 atp per glucose molecule to 2 and the plant releases alot of acids that will skew pH, by time symptoms present there could be a myriad of visual symptoms, nutrients stop uptaking, symptoms appear. Soon as a plant goes aneroboc you lose all bulk flow too, which often explains cal and mag since a large majority of cal and mag would under normal circumstances uptake for "free" (without atp cost) Soon as bulk flow stops every single atom requires a atp cost to uptake, but side the plant went from 40 to 2, very very little is possible now. Good luck.

Could be a PH issues, but its mimicking what I know of MG issues. There are other things that can mimic this same symptom. Yellow between the veins of the leaves. https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg This leaf chart can be helpful. give it a look see. and check your PH/feeding habits.

Looks like nitrogen deficiency. The older leaves are yellowing while the veins stay green, and new growth looks okay that’s classic N deficiency. Give her a nitrogen boost either bump your veg nutrients or add some extra N if you’re already in flower. New growth should green up in a week once you feed.