It may well be, that is all outside my personal scope of knowledge.
However, CMUD is no longer being actively developed, so it is unlikely to be changed.
I am unsure if there is anything you can do ...
I suspect it could be the way it renders the colors. It's similar to ZMud and the colors between ZMud and CMud look to be the same (across 4 computers dating from 2003 actually) but between Mushclient ...