Tried few more times with maroon on Libera and I found out that it happens INSTANTLY as soon as you click ALT+K (all the nicklist colors change instantly) IF there are other settings with the Auto coloring for all nicks, if that's the only option they don't change.

Thanks unfortunately I have still not been able to reproduce this.

If you install a clean, portable copy of mIRC in a new empty folder, with no scripts/addons/etc., and then describe the exact steps you take to reproduce the issue, that could help me track it down.