Entities in your world can randomly freeze when your player gets killed. It is also possible for an entity to freeze when it takes damage. Endermites spawned from enderpearls and chickens can have this problem. Leaving the world and entering it fixes this problem.
Here are some notes about this bug:
Most entities cannot attack (except for slimes and guardian thorns)
Entities will not move and float in the air
Entities cannot burn and undead mobs will not burn in daylight (MC-134965)
Entities ignore damage that would otherwise be non-fatal
Entities can be selected with commands
Entities ignore peaceful difficulty (MC-135361)
Creepers do not explode with flint and steel (MC-135330)
The @e selector can sometimes not find the player and some affected entities (MC-135026)
Entities possibly not counted toward mob cap (MC-135016)
Potions will have no effect on them
Items and projectiles can be desynchronized and cannot be picked up (MC-135256)
Falling blocks and items repeatedly fall (MC-135081 & MC-135089)
Placing redstone dust has weird behaviour (Probably only happens in the world below after /kill)
To reproduce:
Load this world
Kill your Minecraft player (/kill @s)
Click the "Respawn" button
→ Entities will be frozen even ones that have been spawned and the above will happen.
or
Summon an entity
Hit it multiple times
→ Eventually this bug will happen to the entity you have attacked.
Related issues
is duplicated by
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Duplicates MC-123404.
Not a duplicate of MC-123404. This started happening in 18w30a and MC-123404 doesn't talk about entities.
Edit: Also F3+A doesn't help, my frame rate is good, and this can happen to some entities without being killed it's just a 100% chance when you get killed.
Most likely related or cause by this problem:
There is currently an issue with explosions where some blocks will turn into items that cannot be picked up, and some blocks will turn into falling blocks that never set properly on the ground. If you log out and log back in, you will be able to pick up the items and the glitched blocks should be fixed.

I also had "ghosting" while killing Drowned. Logging out, logging in appeared to fix it.

This just happened to me, but I didn't even die. I was attacking a zombie and it just stopped working, but other things work fine (my wolf, a nearby skeleton).
EDIT: F3 + A did not fix, but reloading world did

Sometimes the entities (including items) do not need to be hit, just existing can cause this bug. I've had it happen with dropped arrows, as well as random monsters I see. They also stop making any sounds

Same problem, as described here with guardians MC-135075
Guardians will freeze, and take no damage.
[media][media]

I am also seeing this problem. It is not happening when the player dies though. When I attack a mob, it sometimes freezes after I hit it. I am also finding that if I kill the mob, I can't pick up whatever it drops. If I restart and go back in, I can pick up the item. Both are happening quite often.

On a server, without a restart, there's no way to remove the entities. With a restart, it's about half-and-half if the entities will be affected at all.
With a mob grinder, for instance, some of the mobs become active again after the restart, while some are exactly as they were.
Anyone have a workaround, or are we waiting on a future snapshot? Really makes you wonder what they're changing to affect something like this though.

The best relation I can make is that it is almost like going into the entities' data and turning NoAI to 1, as the behavior seems to be the same (until relog). Undead mobs don't burn, entities float and stop making noise, children don't grow up, items don't follow the 5 minute rule, and no damage gets taken, although that does not seem to be the case.

Also, I should make a better note on my first comment, as to say the entity stops movement before you hit it, but hitting it creates the inability to take damage and etc (confirms the state as I call it).

Didn't have to die, but mobs, whether peaceful or not, will just freeze after an interaction. This is single player. I have to save and close, then reopen to get the mob to move again.

Seems to be fixed in 18w31a, can somebody confirm?

Not fixed.

My 18w31a world also seems fixed. If it's indeed not fixed as Kumasasa says, then maybe it's gotten more rare?

Normal in 18w31a. At lease, cows can move and turn. Fishes can swim happily.

When you all insist that this issue is fixed so let it be.
But seriously: My comment as of yesterday "Not fixed" was based on a comment of a dev who still could repro that, but most probably that was on a world which was brought in a weird state by this issue.
Let's define: This issue is fixed for new 18w31a-worlds or worlds of 1.13 and 18w30b worlds should be trashed.

I have a world created in 18w30b, and 18w31a definitely fixed it for me. I'd get 1 out of 5 entities or so freezing, (at least it seemed to be that often, I was always reloading the game), and now with 18w31a and the same world, it hasn't happened once. And I've killed probably 75+ pigmen already.

Like A. Mouse, my test was a 18w30b world exhibiting the bug frequently, loaded in 18w31b. Not a fresh 18w31b. Well also fixed in fresh 18w31b of course.

I have something similar, but relogging in, F3+A, doesn't work, only /kill works. MC-135256
By the way, 18w31a did not fix it, I don't want to, that world is 6 months old now, from update 1.12, do you really think I will trash it?? Besides, I tried backing up it, it did not back up (1.13) so I have to "let it be.

There was issue with entities not ticking in some chunks after player died around spawn location. It should be fixed in next snapshot.
Exact conditions and affected chunks probably shifted due to other fixes and changes.

I know that this says fixed but I literally experienced the whole world mob freeze thing for the first time in 19w12a. I was messing with the villagers in my creative world, default generation, and I right clicked on one and noticed it didn't open the inv so i thought it was a nitwit. Turns out ALL of the entities were frozen and I could still move around. It was raining and I flew around slightly. When it thawed, it opened the inv and I noticed the trades would dupe my items. Also, i retreated back to another part of the world, tried to anyway. I flew right over the edge of the world into chunks that were not loading at all. Render distance 8. Still not sure what caused it. Significant frame rate drops.
Thanks for trying to fix these bugs.