I have a 4 foot tall cluster of 3 that I bought in November. It's grown about 4 new leafs since then, and the growth seems to be accelerating after I repotted it into a 15" pot in April, with 1 new one this week, and 2 more in very tall tubes about to open. Except since around March, I've been noticing more and more brown spots on the leaves - these are tiny, thousands of them, evenly spread-out, almost morse-code like and aligned with the cell structure of the leaves. None of the new leaves nor the small oldest ones have this problem, but all the others have it. The newest leaf is smaller than expected, has an unusually-ripply edge, and ribs/veins that are closer together than the last few leaves.

I was told it was because I overwater them (about once a week), so I reduced my watering to about every 10 days (about a litre each time). When I do the finger test, the top soil seems dry before I water again, but I would think it's still moist further down (I sealed the drainage hole because I didn't want an unsightly saucer). Not sure if the last part had anything to do with it, because the problem began before I repotted it and my old pot drained into another pot. Any ideas?

FYI, the plant sits by a sun-soaked floor-to-ceiling glass window in Toronto, facing south and west. I leave the blinds partly drawn because I feared that the sun was burning the plant.