Weirdly enough Phoenixmon didn't get the same treatment, his helmet looks fine on everywhere else. In the end, I think the decision is up to the artist or art director for that game or series
In the original art, Plesiomon has a heavy shading on his forehead but this accidentally creates a new black pattern that is present in all his other appearance from then.
It's a bit out of place for his color template but It grew on me.