Health cannot be changed on anything that isn't a player

I was trying to make a zombie with more health than normal (specifically more than 20) when I found out that I couldn't push it past 20 no matter if I used attributes, the health nbt, health boost or armour modifiers. Using {Attributes:[{Name:generic.maxHealth, Base:25.0}],Health:25.0f} doesn't work and nothing else that I've tried works.
Health boost also doesn't work on pigs and other mobs.

I've set up a scoreboard to check how much damage I am doing to it and it won't go past 200 (20.0) which means that the zombie only has 20 health. I suspect that this will be fixed in later updates, but for now, it does not work.

The pictures below show the zombie with the armour on, the armour's attributes and the outcome of me attacking.