Hi Evan and thanks for your question and pictures of your suffering boxwood. I actually see signs of at least three problems, including your leafminer diagnosis. It also looks like there might be spider mites (the tiny white dots and webbing) and powdery mildew (the dead brown leaves).

There is also a possibility of boxwood blight which has arrived in Oregon, but your plants seem to be dying from the top down and the blight usually moves from the ground up.
https://pnwhandbooks.org/plantdisease/host-disease/boxwood-buxus-spp-boxwood-blight

My suggestion would be to prune out the dead areas and give the plants a hearty spray with neem oil, insecticidal soap or horticultural oil to kill any insects. Then read up on boxwood care in the attached site to see if there are other things you can do to help keep your boxwoods healthy and less stressed.
https://boxwoodsociety.org/abs_care.html

I hope this works... the weather may be against us this year!