Boss Bar stays for some secons after switching from world/server.

When you are in fight with the Wither or Ender Dragon and switch over to another world/server, the Boss Bar (with for example Wither, Ender Dragon or any custom text), from the previous server/world, will stay for some seconds, before it actually disappears.

This needs to be fixed, because it's a bug. I have been noticing this since 1.4 or something