Raiden Eagle

I think the problem is that mobs that normally despawn because they are too far away from the player (fish that are not named or from buckets, most unamed hostile mobs with the exeptions of things like illagers and shulkers and stuff like that, unamed dolphins and squids, ect.) will not despawn if they are outside of the players loaded chunks like they are ment to. That is why, when you first create a world in 1.13+ (or in this case, when you load a world in 1.13+) mob spawning is somewhat lower, and overtime as you explore the world and load and unload other chunks, mob spawning gets lower and lower. The mobs in the players unloaded chunks do not despawn, similarly to dropped items (which is intended. Dropped items are ment to not despawn if they are in an unloaded chunk so you have time to get them). This causes the worlds mob caps (hostile mob cap, aquatic mob cap, ect.) to decrease the rate of mob spawns to keep the amount of mobs from going over their limit. I have seen this happen in a creative world I created in 1.13.1. It got so bad and so laggy that now I just play on peaceful mode, but aquatic mobs still make things a bit laggy. There are multiple ways to test this therory. One way is to go into a creative singleplayer world and spawn a hostile mob in a small cage, with a top on it if you are spawning a mob that can burn in sunlight. Do not name the mob or give it any items. Then, fly so far away from it that the chunk the cage is in is no longer in a loaded chunk. Switch to peaceful mode, then unpause the game, then pause the game and switch back to any difficulty other than peaceful. Go back over to the cage. The mob should still be there, and if it's not do the test again but go further away. Switching to peaceful mode is ment to kill every hostile monster (exept shulkers, illagers in unloaded chunks, the Ender Dragon, stuff like that) but it did not kill the mob in the cage, like it would in 1.12.2. Another way to test this is to go into a singleplayer creative world, go into a biome where strays don't naturally spawn, create a cage with a top on it, if it's not already advance the time to the begining of the night, and then spawn a stray. Fly away from the cage until its chunk is outside your loaded chunks. Wait until there is enough daylight that all of the surface monsters have already burned or despawned (a good time would be the middle of the day), then fly very quickly or teleport to the stray in its cage. You will see that the stray has not despawned because it was outside of your loaded chunks, and also other monsters that might have spawned near the cage are also still there and have probably only just started burning once you loaded the chunk. Mobs that don't despawn like this one still count for the hostile mob cap, even though they should just despawn, lowering the amount of mobs that spawn in your loaded chunks. Also for the command you can type /kill @e[type=zombie,type=skeleton,type=creeper,type=drowned] and you can keep adding ",type=" things for what you want to kill.

I really hope Mojang fixes this bug because it ruins the point of survival when hostile mob spawning is this broken.

Also, on superflat worlds that are set to generate with Strongholds, /locate and Eyes of Ender do not work.

I have seen this in release 1.13. (I had photos but they didn't load correctly)

I would give the seed and where I found these, but recreating a world with the same seed will break trees in different spots. This bug was also reported as part of MC-125007 and that bug was listed as being fixed in 1.13-pre10. It seems it was not.