If I changed any of a slime's NBT tag ,including Health
the slime's Health will be reset immediately
Linked issues
Comments

I use "/entitydata @e[type=Slime,c=1] {}" to check a 3 size slime, and the Health is 16.0
then I use "/entitydata @e[type=Slime,c=1] {Health:1.0f}"
but when I use "/entitydata @e[type=Slime,c=1] {}" again to check its Health, it's still 16.0
I can reproduce the bug. Furthermore the "/entitydata @e[type=Slime,c=1]
{Health:1.0f}" commandfeedback displays Health:1.0f for the Slime.
Fixed in 1.11.
Please provide the exact command you've used.