Leaf pic - what's my issue

Right now, I have 2 strains going (Kripple Shock and a CBD), two plants of each a week into flower. All get the same nutes (Dyna grow, Mag plus, ProTekt ), soil, etc. All in all they look good but on one plant of each of the two strains I am seeing some of this on old growth, new growth looks fine. The other plant of each strain is fine.

Seems weird to me. I could see if one strain got it because of different needs vs the other, but why one plant of each strain?

check out the chart in the 4th post of this thread
https://www.rollitup.org/t/please-help-dark-spots-that-are-spreading.866015/

Awesome chart chuck! But not sure where it leaves me. MG would be my guess???