hey i just seen your message. im not the weed professor like the others here but my guess is that its a problem with the soil. maybe a ph problem.
i have a couple patches out the main one is a pretty big spot by a river that i turned myself, just digging out old roots with my shovel, no soil bought or mixed or whatever i just turned the dirt and planted in there. they are all doing fantastic with the exception of some plants along the perimeter (those closest to other plantlife, and furthest from the center) they have this same problem with the yellow spots. so i assume yours is the same problem and therefore a soil problem.
i have a couple patches out the main one is a pretty big spot by a river that i turned myself, just digging out old roots with my shovel, no soil bought or mixed or whatever i just turned the dirt and planted in there. they are all doing fantastic with the exception of some plants along the perimeter (those closest to other plantlife, and furthest from the center) they have this same problem with the yellow spots. so i assume yours is the same problem and therefore a soil problem.