I'm going to take a stab at this and suggest it might calcium related as well. I'd be expecting to see a mg deficiency look worse further down the plant, on the lower fans. But lookouts can appear different. Calcium and magnesium go pretty hand in hand. If your plant is deficient in one, then it's probably deficient in the other as well. Potassium deficiency / lockout will probably soon follow, or has already started too.
It's a small pot, so i'd rule out a lockout first off and run to waste when you water. Salt accumulation could be causing your problem.
Because it's a small pot i'm doubtful it's a ph problem, or overwatering, but it's still within the realm of possibility.
If all else is failing, consider an organic calcium source like dolo or gypsum.
Best of luck.