This happens frequently around me in southern coastal new jersey. I have a very large specimen my pop worked on since 1980 and then I've taken over since early 2000s. It just started the past five or so dying back up top in center. Gave a it a full crown reduction , light dose of Green tissue removed , and it held up in regards to not dying back significantly more until this year. I will be going there this week coming to remove the large dead center lead around 20 foot and will get some pictures. I've other trees same species Acer rubrum that are not old or large that lose center lead tips first more so than other leads. I have another one I'm thinking of I'll get the same day with some pics as well. Verticulm wilt in my experience shows flagging on a whole side lead albeit upright in orientation usually. All tree are so different , not so much physiologically, but in there physical location on this earth they endure and adapt to survive any way they may based on their specifics... shed a limb when you have more is no big deal to a tree I take.