Eu descobri isso hoje usando uma dica de esta discussão .
"Thanks Tony, that means a program has raised the MustBeSeen system flag. Typically this is a program that needs your attention, like antivirus or backup program that has some message it's waiting to tell you. It is almost always a program icon near the clock in the notification area. Once you clear the MustBeSeen flag by addressing the program, the taskbar will hide again.
As a best practice suggestion, hit Ctrl+Shift+Escape and disable anything on the Startup tab you don't need autorunning. That limits how many things load there, and lessens how many programs can raise the MustBeSeen flag."
There must have been something in the Windows 10 Anniversary Update that's changed the way how the taskbar overlays work and so seems to be causing it.
Por isso, fechei todos os meus aplicativos em segundo plano com o ícone de notificação e o problema desapareceu.
aqui é um link para a resposta citada .