Hi, thanks for using Ask Extension

It really doesn't look like a disease or pest issue; however, it is early. This looks a lot like sun damage, trimming stress, or possibly a nutrient issue (low nitrogen or lockout due to high soil pH, compaction even cold soil.  Excess of certain nutrients can also interfere with iron (Calcium, phosphorous et al).   It really doesn't look like a disease or pest issue, but it is early. If this is the south facing wall of the house, reflected light and heat could cause this.

So, what should you do. Check the pH of your boxwood bed, a soil test (Home | MSU Soil Test) can give you this plus fertilizer requirements.   Make sure your mulch is not compacted so air can penetrate and keep the root flare free of mulch.  a sunscreen during winter may help if you have a heat reflection problem.  Keep moisture consistent.  Since boxwoods are susceptible to many maladies such as winter damage, box tree moth, boxwood leafminer, boxwood blight, etc., it is important to learn the symptoms so you can act promptly and preserve your shrubs.

Check out these links:

Boxwood: Identify and Manage Common Problems | University of Maryland Extension

What’s Wrong with My Boxwood? Common Issues and Solutions - Gardening in Michigan

More reasons for soil testing - Agriculture

I hope this helps.