The bug
After changing a tamed wolf's max health with the command:
/data merge entity @e[type=wolf,limit=1,sort=nearest] {Health:10.0f,Attributes:[{Base:10.0d,Name:generic.maxHealth}]}
stays at 20.0d
when checking with data get command and also dies after getting 20 damage, not 10.
confirmed in the newest snapshot 19w40a
not only generic.maxHealth doesn't work, the same goes for generic.attackDamage