Since this happens every year and it is expanding, I believe it is the grass type. In the 1960 and 1970s there was a group of “midnight” strains of bluegrass that was popular. While it does eventually turns a nice dark green, it is VERY late doing so. I drive past one of these lawns every day on the way to work. I expect you have a patch of “midnight bluegrass” that is doing well and expanding. The only cure is killing it, turning it under and starting a lawn using a more modern seed blend. You can send me a picture if you like.