When you summon a boss, the boss bar still shows the same as usual. For example, if you do
/summon WitherBoss ~ ~ ~ {CustomName:"Example",CustomNameVisible:1}
the boss bar says "wither" instead of "Example" when you kill the boss by doing /kill @e[type=WitherBoss] it says:"Killed Example" I'd love to see this glitch fixed.
Linked issues
Comments

duplicate of MC-83571