Since updating my server from 1.14.2 to 1.14.3, the only spawns I get are the following :
Phantoms as previously said by another user--
Illager patrols
Zombies from Zombie siege
Illagers from Raids
Mobs spawned via Spawner
I'm 90% sure that "standard natural spawns" do not happen since I got a big slime farm and it does not produce anything anymore.
I found a workaround (mobs started spawing anew) by installing "paper spigot" still don't know what went wrong.