Sick Japanese maple
Japanese maples are subject to many environmental stressors such as drought, poor drainage, too much moisture, temperature extremes, too much mulch, planting too deeply, soil compaction, etc. In addition, with Japanese maples, the bark can freeze and kill the cambium which can cause dieback. These are all possibilities for the branch dieback you see.
You can prune out dead wood at any time. Make sure mulch is no deeper than 2-3" inches and keep it away from the base of the trunk. Keep the tree well watered during dry periods. There is not mulch else you can do.
Please see the following article by our plant pathologist. https://marylandgrows.umd.edu/2018/08/24/japanese-maples-in-maryland-landscapes-plant-location-care-...