Can you confirm what you're doing in the Alt+K dialog? Are you just looking in there and saving no changes? Or, has the background color changed to something else?
If you're changing the background color's RGB, that can cause colors to change. The number of colors used in the random-nick color palette depends on the RGB of the background, because the number of colors depends on how many other colors there are in the color list who have sufficient contrast against the background. And, if the number of colors in the palette changes, this causes everyone's color to change, for the same reason it changes everyone's color in the earlier MD5 script when you add/remove a color in the script's list-of-colors. More details on how the color palette is possibly chosen is at:https://forums.mirc.com/ubbthreads.php/topics/266256/re-automatically-colourize-nicknames#Post266256
... where I'm finding that there are multiple combos - at least for background black - where the threshhold of contrast for allowing a color into the nick palette is lower than it should be. There are some shades of blue which are allowed to be for a nick against a black background, and while you can discern them from each other when your eye is right next to the screen, the contrast disappears at a normal eye distance from the monitor.
And I backtrack my earlier guess about the address being used for coloring the nick. It appears that a case-sensitive version of the nick is used for the color, as I see the same color used for the same string at 2 networks where their address details are different, and the nicks are continued to be colored identically regardless whether or not I've used /who to even discover what their address and/or account info is.