Fight between mobs stops after quitting the world

What mobs?

Was able to re-create this in Minecraft 19w14b.

@Neko: I tested multiple mobs;
-Skeleton vs. Skeleton (Stopped When I left)
-Zombie vs. Skeleton (Stopped When I left)
-Blaze vs. Skeleton/Zombie (Stopped When I left)

Confirmed. Do you know if this issue occured prior to 19w14b?

@violine1101 it happens between all hostile mobs,for example zombies and skeletons,with exception of ghast,guardians,elder guardians,slimes,magma cubes because they don't fight back other hostile mobs.
if you don't believe ,look at @Thomas Dorland's comment

@ violine1101 ,when I made pillagers fight drowned,the fight stopped when I quit.

Can confirm in 1.16.5 and 21w08b.

Can confirm in 1.17-pre4.

I believe this ticket would now be considered a duplicate of MC-250059. Despite this ticket (MC-147628) being created at an earlier date than MC-250059, MC-250059 has been triaged and has assigned issue links.

I suggest keeping them separate, as this report involves old-AI mobs, while that report is new-AI focused (warden); they will both require different fixes, since the new AI just needs to write/read the memory to/from the Brain NBT, while old-AI mobs never use Brain, and use a vastly different data structure.

Confirmed for 1.19.2.