macOS 10.12.1, JRE 1.9
Spawning a wither displays a bossbar showing the text "Wither", as it should. Renaming the wither using a name tag should update the name on the boss bar but it remains "Wither". Reproducible on both singleplayer and multiplayer. When you re-enter the world the boss bar will be fixed.
- relates to
MC-83571 Custom named bosses show "Wither" / "Enderdragon" at boss health bar