You can fix simply by extracting "Units\Human\Arthas\Arthas_portrait.mdx" from the MPQ to the same destination as the "Arthas_Weaponless.mdx" file, then rename the extracted file to "Arthas_Weaponless_Portrait.mdx"
If a model file doesn't have a portrait camera, "Portrait" animation inside it, and no separate portrait model file, then the portrait screen will flicker in-game.