Enemy castbar is showing on all mobs nameplates

[mutantsanya]

When one mob is casting something, I see the castbar on all the mobs nameplates around me.

[shagu]

It can't be done any better in vanilla. The enemy casts are detected via combat log messages which don't show a unique id per mob, but just the name of it. So everyone with that name will get the cast bar attached to it.