The brown patches in the places with sectorial variegation (variegation where there's largely a big patch of white) are normal. These patches have no chlorophyll, and will start to break down before any green parts or heavily variegated parts of a leaf will.
Many people try to 'stabilize the white' by using Cal-mag supplemental fertilizer and silicon. It doesn't really work. The sectorial variegation is just always going to brown and deteriorate.
I can't tell where the petioles of your leaves are...the leaf that is yellowing, is that an old leaf? Were the ones you lost older leaves?
