According to the tech at my dealer's it is an issue of too little glue applied to the cover.
On the first 2 occasions he fixed it according to procedure, the 3rd time he went nuts with the glue and it never leaked again.
That's exactly right.
If it's leaking at the D's on the gasket, then it's a sealant issue and that all comes back to the mechanic that's doing the warranty work.
Perhaps the mechanic needs to be retrained.
Or you need to find one that can put a rocker box gasket/seal on properly.