Sorry your question went unanswered. I'm not sure that I can help your plants to recover but I can say that no, Buxus, at least the ones I am familiar with, should not lose their leaves during the winter. I have several in cold storage right now that still retain their foliage. They do change color and become a darker shade, almost a bronze tone, but should not behave as deciduous trees do. You have some other issues.
Does anyone else have anything to add?