I bought a giant bird of paradise (white bird) mid December 2019 and lately a few of its leaves have been developing yellow spots and then having those spots turn slightly brown, while also getting a little dry. The plant is in a western facing window and because of the overhang from the apartment above it gets direct sunlight from about 4:30 till just before 7 pm, when the sun disappears behind the building on the other side of the street. It's made up of three different stalks. In the time that it has been with me, it has put out six new leaves, one of which (actually, the first of the six to sprout out) has the worst yellow and brown spotting of any leaf. Of the other five "new" leaves only one of them has spotting, but it's very mild.
The picture below is the day I bought it home, mid December 2019
and this picture is today, 4.5 months later.
Below I've included some pictures of the worst offenders but I also offer pictures of some of the healthy leaves. Could the yellow/browning be too much water? root rot? Once i noticed the coloring i scaled back water from once a week to once every ten days. I also did a flush of the soil about a week and a half ago because i thought maybe i had over fertilized it. it's odd that one stalk in particular has two leaves with the worst of the spotting.