I personally do not believe this is a cal/mag issue. Especially because it is the lower leaves that are developing these symptoms.
From my understanding (anyone correct me if I'm wrong) cal/mag is an immobile element. So if you had a cal/mag deficiency it would appear on the newer growth.

When you have a deficiency with a mobile element the symptoms first appear in older growth, and then work it's way up.

From your pictures and descriptions it looks like a potassium deficiency to me.
Since you are using the MG six month, I would recommended doing a complete flush with ph'ed water. This should help pop some of those nutrient capsules in the soil and make sure you don't have any dry spots in you soil.

I am by no means an expert and this is just my :twocents:
Best of luck and happy growing. :hippy: