If you spawn in a mob (with /summon (that's what I did)) then giving it an effect (with /effect (also what I did)).
and then do pickblock+Ctrl

(for example
/summon Creeper ~ ~ ~
/effect @e[type=Creeper] 19 10
now ctrl+pickblock on the creeper
NBT tag is not copied, but you get an regular spawn egg... without effect.
)

maybe this is intended cause of ingame reasons. but maybe it is a bug and it should be fixed. please take a careful look at this.