Illagers will not accept modified NBT Health

Well, I attempted to make an interesting boss fight using the Illusion Illager.
Thing is, the Health on it, and the other Illagers will not change no matter what I do.

In 1.12.2, the /entitydata says it works, but checking, it doesn't.
in 18w01a, it says the /data merge entity works, but checking, it doesn't.

This is very sad.