Hard to say. Could be the engine is running lean, could be there is deposit buildup around the valve seats, could be inadequate cooling, ... Once they actually take a look at the valves, they will know better. May not be just the one, may be others on their way out as well. It IS true that a tune runs leaner. In order to make more power with the same fuel, that's one of the parameters normally adjusted. But hard to tie it down to that, as there is literally no data/anecdotal evidence on this type of event in (at least) the transverse 3.5EB.