What happens when a Buxus Sempervirens has dry patches on leaves
My Buxus are living in a southwest facing windowsill, and one of them is having a growing patch of yellow drying leaves.
While it's not unheard of to use boxwood as an indoor plant, neither is it common. I don't think boxwood adapt easily to the restricted roots, weird drainage, and lowered light that they find inside houses. Because of this, they are going to be easy pickings for any number of diseases and insects. You will have to examine the leaves (undersides especially) and branches for bugs: https://www.gardeningknowhow.com/houseplants/hpgen/common-bugs-and-pests-on-houseplants.htm
Then look for any kind of spots or discolorations on the leaves adjacent to the brown area. This would signal some kind of bacterial, viral, or fungal infection. For many of these there is no treatment other than to cut out the infected area before it spreads.
There could also be a broken area on the main branch.