Why Scharnhorst's shells need not repeat River Plate failures

Would Scharnhorst's more powerful 11-inch shells suffer the same failures seen in Deutschland-class ammunition at the River Plate?

Not necessarily; the River Plate ships carried earlier shell production, while German ammunition design and manufacture continued to improve.

The River Plate problems are linked to older shells carried by the Deutschland-class ships. Scharnhorst's ammunition had its own limitations, but it came from a later stage of German development. The useful distinction is between a weapon caliber and the changing production quality of the shells fired from it.