Been making a map recently and I've noticed a very strange bug. 
Any time you use a structure block that contains an under dragon within the structure; if you load it, three extra ender dragons will be summoned at x:0 y:0 z:0.

The ender dragon within the structure block has a tag called "mapdecor"
and also has {NoAI:1,Silent:1,Invulnerable:1}