From what we can see in the photo, the shrub appears to be in good condition overall. The yellowing leaves are probably part of the normal shedding cycle that evergreen plants do yearly. (In boxwoods and hollies, for instance, it's in spring; for other evergreens, like pine and azalea, it's in fall.) While evergreen leaves do live for several years, they do have a finite lifespan and eventually shed. Dropping leaves will tend to be the lowest and innermost ones, but on an upright-growing, nearly-columnar variety such as this, their positions can be a bit more scattered and obvious because fewer branches cover it up. It is harmless and a normal phenomenon which should slow and stop in the coming weeks.

Leaf yellowing and shedding can also point to root health issues, but the younger foliage on this shrub looks normal, so that is not a likely explanation here.

We do see moderate levels of boxwood mite feeding damage, though it could be from prior years. The mites are so small that we cannot see if they are currently present. This page gives more information about foliage mites in general: https://extension.umd.edu/resource/spider-mites-trees-and-shrubs

If opting to treat now with horticultural oil, be sure to use a "summer rate" (unless a bottle is labeled "dormant oil" it's probably this summer rate, which is less concentrated to avoid plant injury). Thorough spray coverage of upper and lower leaf surfaces is important, which is challenging to accomplish on dense or large shrubs. Repeat treatments may be needed - check product label instructions. Do not spray during temperatures above 85 degrees to avoid leaf injury from the oil itself. You do not need to spray for the mites, but monitor the plant for worsening symptoms on its tip growth. Usually, boxwood mite is not a serious pest that impacts overall boxwood health; it's damage is more cosmetic and subtle compared with other potential pests and diseases.
https://extension.umd.edu/resource/boxwood-diagnose-manage-and-prevent-common-problems

Minor winter injury (which includes late-spring cold damage after the plants broke dormancy) could also result in the slight off color to the foliage when compared to the other shrub. Malnourishment is possible, but in that case it would be more likely due to ailing roots than an actual deficiency in the soil. To avoid this, make sure the plant is well-watered as needed during drought but otherwise not getting extra water, since boxwoods are sensitive to soils that don't drain well and are excessively wet. Sometimes, surprising moisture and drainage differences can exist between plants just a few feet apart. The minor discoloration on this shrub is not alarming at present, so for now, don't fertilize but keep an eye on soil moisture (at a few inches deep) as we move into summer.

Miri