I used this command:

*/summon slime 92.5 101 108.5 {NoGravity:1,Size:0,Silent:1,NoAI:1,Health:20,Attributes:[

]}*

I found out that this slime (perhaps other mobs, but I only found slimes having this issue yet)
will immediately drop to 1 hp after I hit it with anything.
I tried to hit it with a fist and a diamond sword with no enchants nor status effects, it just shows a lot of heart particles dropped from the slime, but survived. However, the next hit will just kill it, same for fist and sword.

I tried to increase it's Size to 2. Again, a lot of hearts dropped, but survived from a fist or diamond sword. After that, it'll just be killed with 10 hits by my fist. The split slimes act as normal slimes (And of course, NoAI resumed to 0 as well, but unimportant for this issue).

One strange thing is that, if I get a creeper to explode next to them, they didn't survive. I guess if the damage exceeds the slimes' "shield health", it'll just die right away. Otherwise, the "shield" will just go off and the slime returns to normal health.