If it's still under warranty, submit a claim. They will respond one of two ways.
1- tell you it's an adjustment and how to fix it.
2- authorize a warranty return.
This is something you should have done before doing anything beyond an external adjustment or cable replacement. Hopefully, they won't use the fact that you opened the hub as an excuse.
Either way, opening a dialog with a tech rep at the company is your best bet.
Otherwise, slippage in a single gear is usually either a clutch or pawl issue. However, I'm not comfortable with my own knowledge of the torque path through this specific hub, and can't give you more specific advice. Hopefully Dan Burkhardt will see this, because he's much more intimate with Nexus, and other Shimano IGH hubs than I'll ever be.
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FB
Chain-L site
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