I had the same issue on two machines.
After several reboots, I went into the "show hidden icons", section of the lower right tray, and pulled up "customize".
At the bottom I saw a defunct? (non editable) Avast icon and a new Avast icon on the top. I edited the new Avast icon to "show icon and notifications" which fixed the issue.
On another machine, I had to drag the new icon from customize onto the tray and it has stayed there since.
Hope this helps... I see some one else had an icon issue.