This is the story:

I spawn two ender dragons,one spawn by crystals(nature),
another one spawn by command(op).
In the first part of battle,there's just two health bars,one is nature,another is wither
(I let a wither ride on op by command),and in a dragon-perch,this happen.
Nature perches first,and op wanna perch too,but nature keep knockback op,
and a incredible thing happens:
Nature is gone,but op GET THE HEALTH BAR!!!(I just have place ONE command block,to spawn op)
I am full of question marks with this,plz tell me how this happend.
(Image 2 tells nature is actually "eaten" by op)