You know from looking at the large leaf. I hope you are right about it being some bugs.
Are you using RO water?
Do you use CalMag Plus?

It could be mag defenicy as well rust spots are a sign of that.

FYI
Manganese immobile
The toxicity is compound by low humidity. The low humidity will make the plant transpire more and draw even more manganese out of the foliage. If it keeps up there will also be deficiency of Fe and Zn as well.