Yellow Warning Triangle - View Settings: Layer Group

[Stress Co_]

I have a bunch of saved views, that when opened from the View map,
display a yellow warning triangle next to the layer combination.

The warning is:
"The settings in the frontmost window no longer correspond
to the original view settings."

The "Quick Options Bar" shows the layer group as "Custom", even though the
saved view has a definite layer group selected/saved.

Note that this is prior to even clicking in the newly opened view window.

Can anyone think of a reason why?