Non-default disabled GKC image seems to be ignored or greyed out

Disabled GKC button still displays the default greyed-out image when a specific “disabled” image is specified.


Results in this, as if the “Button icon when disabled” had been left blank, or perhaps Vassal is displaying the correct image but greying it out anyway:

Expectation was that the colour image would have been presented even when gOptionTip = true.