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.

This:

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:
image

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