All versions of mIRC use the same FlashWindow() API as that is the only method by which an icon can be flashed in the taskbar, as far as I know.
I tested the /flash command you provided in your post on mIRC v6.35 and v7.52 under Windows 10. In my case, sometimes v6.35 flashed and then became a solid color, other times v7.52 did it. It is completely random and I have seen this issue ever since this feature was added. Even manually calling the FlashWindow() API, eg. if I create a /command just for that, to reset it does not work. It's a mystery.
