Mobs spawn but in such low numbers you have to hunt for 15+ mins in an unlit open field to find any hostile mobs. Fish and squids are also no longer spawning. In the picture attached the pond had 20+ squids spawning in it constantly, until server was updated to 1.13 pre3. I could walk away and come back and the squids would be repopulated. The server world was created in pre1 and mobs were spawning like normal until updated to pre3.
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What version was this world created in? Also, what versions was this world upgraded to?

The world was created in 1.13 pre1 and has been updated to every snapshot since then.

Is the mob spawn rate normal when you recreate the world in 1.13-pre5?

By recreate do you mean update the server to pre5? If so then no it didnt affect anything.

No, he means creating an entirely new world using the same seed as this world has.

Yes, that's what I meant.

I never recreated the world, but I found a fix to increase mob rates. For some reason mobs don't like to spawn when the render distance is set to anything lower than 10 on the server. Once I set the render distance for the server at 10 mobs started spawning more often.