Yes, it is. Primary display is OK. It's white on 2nd and 3rd displays.
I haven' tried removing Start10 so I don't know whether there is a change to the standard start button behaviour.
I have reproduced it here. It is happening because of a small OS change on the secondary taskbars. We have it fixed internally now and will put out an update with some other tweaks next week I suspect.
I have also seen it without Start10 with a vertical taskbar in certain circumstances for the buttons below the start button, so it isn't confined exclusively to us but it is worth fixing.