We already did Artemis I and the heat shield lost a lot more material than it was supposed to on that flight. "Specifically, portions of the char layer wore away differently than NASA engineers predicted, cracking and breaking off the spacecraft in fragments that created a trail of debris rather than melting away as designed. The unexpected behavior of the Avcoat creates a risk that the heat shield may not sufficiently protect the capsule’s systems and crew from the extreme heat of reentry on future missions."

Fixes have been made to the design, but they haven't been tested in flight.

Also the fixes weren’t made on this capsule, since it was already built with the old design.

So that means this capsule will fly a different re-entry profile to attempt to avoid the issue and Artemis IV will fly with untested fixes for lunar return.