Most likely your Box Turtle died from impaction or some other trauma/complication from eating meal worms. As was previously stated, the cuticle can be a problem to digest.

The soft plastron would go unnoticed while alive, as this is the softest weakest part of the shell at that age, due to the forming/yet unformed hinge. Kinda like a baby's soft spot.

I feed my neonates earthworms, baby crickets, small grasshoppers snails/slugs, butterworms, other soft-bodied grubs, and any fruit or vegetable matter they may take. Keep a watchful eye out for ants. They can be a problem.

Many people raise their box turtles in a semi-aquatic environment with half their enclosure consisting of a shallow water element. As in tilting the container slightly (15 degrees or so) with the lower half wet. Box turtle should always be able to touch bottom, even at the deepest point.

Always provide a burrowing medium. A latticework structure just slightly above the substrate provides a secure and psychologically appealing place to bask.

jsc

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"As hard as I've tried, just can't NOT do this"
John Crickmer