>>13510658I guess you're refering to the Delta Species pokemon. There was a story reason for this (which wouldn't be canon in the games or anime). This also a different circumstance than the physiology of pokemon.
GameFreak supplies the teams of the other creative teams with a bible for stuff like how a pokemon appears and works via reference sheets like pic related. This assures that a pokemon is represented accurately throughout the various medias and stories.
An art director would have to pass off on every piece of art and make sure a pokemon is as canon as possible.
In pokemon's early years, something like this wasn't in place, but now-a-days it's a must. Especially when teams are now expanding and working closer than ever, which yields us things like quicker localization.
In the TCG, there is more than one Drifblim image showing its mouth open, so it seems unlikely that a mistake was made.