Yeah, I have a similar employee. Pretty much no matter what task I assign him, unless I spell out down to precise detail how to implement it, he will go off-stack to do it. Or if he stays on-stack he will insist on completely refactoring the code, creating his own library (which several times he has done and then asked to open source) or in some other way dramatically upending whatever code he is working with. At this point I'm not sure what to do because he is the most productive employee in terms of simply "getting things done", but I consider him almost dead weight in the sense that he creates almost as much technical debt (in an odd sense because its often forward looking - using tech newer and more fancy that what we are running, or just disrupting code everybody already understood so everybody has to relearn it) as he contributes usable code with everything he does. A couple of times I've forced him to go back and rewrite using our stack but it doesn't seem to be changing his behavior.