hey Frondo, yeah you're absolutely right: it's mag deficiency. two ways to get it fixed : add cal mag, otherwise dolomite lime . Check the comment below if you want to know more. Hope this helps ! 🚀
Agree, Magnesium defiency. You maybe had some PH fluctuations. If you mix next time Dolomite lime or Vermiculite in your soil, your Ph will be more stable. Canna got Magnesium as single Product without Nitrogen.