Have you tried typing /legacymark, as that command toggles the Legacy Mark to be visible or not.

Another possibility, what body gear is your character equipped with? Naturally if the body armour fully covers the back, then the Legacy Mark is not visible. However, there has been issues in the past where certain body pieces that have the upper back exposed does not show the Legacy Mark (one of these was the Riveria Dress, until SE patched it). It's possible a new body piece is suffering from the same issue here.