This is related to the (fixed) bug MC-95738

That bug was about Slimes Heath tag being reset after using "/entitydata" or reloading the world.

This problem got fixed but revealed another problem being, you can't change the generic.maxHealth Attribute like you can with other mobs like Zombies.

How I expect it to be (and how it is with other mobs):

• You can set the generic.maxHealth Attribute either directly in "/summon" or later with "/entitydata"
• After that the Health will be updated so it is smaller/equal to the maxHealth Attribute

What happens:

• When you set the generic.maxHealth Attribute for a slime with "/summon" or with "/entitydata" it won't change
• Furthermore, instead of changing the maxHealth Attribute it actually SETS the Health tag to the value of the given maxHealth. With other mobs, how I just explained it only makes it so it is lower/equal than the maxHealth, but here it just directly assigns the value.
• Also obviously this health does reset to the still unchanged (/unchangeable) generic.maxHealth Attribute the moment you hit it (which is the correct way of handling this problem... I guess)

Have fun fixing~