After healing a wither boss over 1/2 HP, it still retains its anti-projectile shield

So I was fighting the wither (in Creative), but I decided to test if arrows of Instant Health II would damage the wither more than Instant Health I, and as I went throwing splash potions of harming at him, he did regenerate, but his shield was still there! Only when I restarted Minecraft did it disappear. Now, Mojang, what?