When Isshiki reincarnated through Jigen, why did Kawaki lose his Karma mark yet Code never lost his?
The variant has never been seen before, but Code breaks down why he has it. The villain admits the White Karma is left behind when the black mark fails to take. A white symbol indicates the wearer is not suitable to become an Otsutsuki vessel, but they can withstand the clan's power.
The White Karma is a variant of the normal, black Karma seal which, as usual, grants the holder the power of the Otsutsuki which placed the Karma, although they do not become a vessel, which is a very rare case. Code was one of the exceptions, and he did not become a vessel of Isshiki Otsutsuki although a Karma was implanted in him by Isshiki.
Kawaki no longer has his karma seal. At least currently speaking. And that is because Isshiki Otsutsuki was forced to choose Jigen as his vessel. ... This is done to prevent a duplicate Isshiki Otsutsuki walking around, so, because of that, Kawaki lost his Karma seal.
Code would not lose his Karma because he is not a vessel, and when two or more people are the vessel of one Otsutsuki, once revived in one vessel, the other Karma will disappear. As Amado explained, this is likely a safety feature that the Karma has to prevent duplicate personalities from being revived.
Code's Karma did not fade because he isn't a vessel of anybody, he just has the Karma as a weapon. It would only fade if he had a complete Karma, not a white one that just amplifies his strength.