Probably not frost damage. Maybe a pathogen.
Out of doors, exposed to the cycles of deepening frosts in fall, it can be hardy to zone 5's subzero (F) temperatures. In a cold, dry, but sunny winter conditions, the cambium can desiccate on the sunny side and is commonly called 'winter burn' - this doesn't seem to be indicated in your pix.
Pseudomonas syringae is a winter pathogen and a possible cause. The bacterium can gain entry through bark damage but usually during freezing by being a nucleation site for forming ice and so is associated with being a winter disease. It doesn't seem to be fatal to larger acer palmatums, but on bonsai size, I dunno.
It also could be fungal - 'flame out' this time of year could indicate verticillium for which there is no cure. Verticillium is soil-borne and gains entry by being 'rain splashed' up into a bark wound. It gets into the xylem and so progresses upward from the point of entry.
There are other possibilities that are less common and/or not so well-known.
The pix are not clear enough for me to tell for sure if there is bark damage on this branch, but none seems to be indicated. I also cannot tell if that bare branch has viable buds. If it does not, it is dead anyway and keeping it is just inviting more trouble, so remove and discard it after doing a little autopsy (verticillium makes dark, almost black streaks in the wood, for example). Be sure to sanitize your pruners afterward (e.g., wipe the cutters with 70% isopropyl alcohol).