Minecraft 1.13 Mob spawn rate

Just had the same problem on the server I'm playing on. Seems like it's caused when players leave the area where I'm at, that's when mob spawn rate suddenly drops. I've managed to fix this by making sure that no one is at the same area and then going into the nether for about a minute, once going back, mob spawn rates seem to be normal again.

I have had the same problem with hostile mob spawns, less than 10 wither skeletons with a 200x 50 block spawn pad over a lava lake in a 2 hour time period. The only mob I consistently find is drowned

While on a multiplayer server (1.13), it appears that once someone signs in and loads up mobs, that when a second player logs in the mob cap fails to split.

The first player will get a lot of mobs spawning around them, but new players will not.

On our server, we have a drowned farm over a river biome.  With a player logged in and in spawn chunks, afk at farm produced exactly 1 mob in 1 hour real-time.

Solo, there were hundreds spawned in a few minutes.

I have no evidence that this is specific to drowned, nor do I have evidence that is not the case.

I'm currently experiencing the same issue with Slimes. I build 6 layers below y40 each 3 blocks high.
I made sure surrounding areas are lit up properly so no other mobs are able to spawn.
Waiting about 30 minutes about 30 blocks away from the layers gave me exactly 0 slimes.
I tripple checked with different sites if it's a slime chunk. I even had a single slime spawning there when building the farm.

Edit:
After checking the farm again for another hour, I had some slimes spawning. About 1 every 10 minutes which is kinda frustrating.

@recon you should try it at least once with just you on the server. This appears to be a bug with how mobs are distributed amongst the players. 

@Hraefin I never mentioned playing on a server 😉 

I concur with @recon; This is most definitely not limited to a multiplayer scenario. I experience the same abysmal spawn rate in both SP/MP with the same world.

Just made a couple platforms at y240 and left the game open about 30 blocks away from it for 30 minutes with time always set to night to ensure low light levels.
About 8 mobs spawned there. 

This might be related to MC-134506 .

I am having this problem in 1.13 Java Realms. I'm getting very sparse spawns (less than 10 in an 8 hour period) in a witch hut that I built, with the area around it made spawn-proof and my idling platform about 95 blocks above it to avoid the possible interference of underground caves.

I read the description of MC-134506 as Thilo commented above, and if this was an intentional change meant to break AFK farming, Mojang needs to come out and say that, so players know whether it's a genuine bug or expected behavior.

@Amanda While I certainly hope that isn't the case, I agree that it would be helpful if Mojang provided some clarity on this issue. Because even if that is the case (expected behavior) then it still needs tweaking. It's far too extreme.

AFK farming is practically a fundamental staple of Minecraft and has been since the beginning. There is plenty of satisfaction a player can get in thinking through strategies on how to utilize game mechanics to optimize resource gathering... in the quintessential RnG game of the past decade. If that's taken away... Many will stop playing.

@Amanda: If this was intentional to break AFK farming, the workaround is to load and unload surrounding chunks. So, hop in a minecart and circle around the perimeter.

These spawn rates are ridiculous. I've been trying to gather ender pearls for about 2 weeks now to re-ignite my end portal since the update to 1.13 cleared the chunks in that area. I only needed 6 or 7 and so far I've only managed to get 2. I wouldn't mind since I have chests full of the things IN THE END but I can't get to them.

@Simon Nemeth - The best way I could recommend for getting Ender Pearls at that point, would be to trade with a Cleric. But yes, I understand. I've been playing 1.13/1.13.1 for weeks now and the number of Endermen I have seen in Overworld, I could count on one hand.

in java edition in a nether fortress the rate for spawns of mobs (wither skel's and other mobs ) are at a dead rate, no mobs spawning at all or initial spawn and then no respawns. the server is in hard mode so why the no spawns/respawns ?  I tried even to go away from the fortress to another fortress and the same at the next fortress no spawns/respawns.

Same problem here.

It's affecting our new installed SMP-Server for version 1.13 and currently updated to 1.13.1. 
We are playing on hard mode.

Except from Enderman only Phantoms are spawning like intended and some Zombie Pigmans, but sitll very low compared to 1.12. For all other mobs the spawn rate is nearly zero.

None of my mob farms are working (only Enderman farm is working fine). At night there are no mobs spawning or at least just a handfull in total within 1-3 hours. Even in unexplored caves nearly no mobs are spawning.

A few days ago our serveradmin reinstalled the minecraft server-software. For a few hours the spawning rate was very fine and all farms were working but then the spawning rate dropped again to nearly zero...

To be honest, the only danger at night are the phantoms... if you don't sleep...

Hope this will be fixed soon.

UPDATE

As in Issue MC-134506 described, mobs only spawn when moving around. I've tested this case with my AFK mob farm (design by gnemban). While standing AFK, no mobs were spawning. After flying around the farm with Elytra the spawning rate increased temporarily, until I went AFK again.

This issue relates to MC-132944.

The summary and description of this report are not especially descriptive - currently, if we want to see exactly what is happening we have to take a closer look at the comments. I suggest linking to this comment and this comment in the description so we have a reliable place to go to see what exactly the problem is.

Additionally, the environment is only supposed to contain PC details - e.g. information about your operating system, graphics drivers, Java version, or anything else that might be relevant. For most reports an environment isn't really relevant since the issue affects multiple devices.

The 1.13.2 update has drastically improved mob spawn rates in my experience (the witch farm I had mentioned in a previous comment is suddenly extremely productive). In my opinion, this issue is resolved. I hope everyone else is experiencing similar success.

My dropper gives me about 2 mobs per ten minutes. I ran it on a 1.11 world and I get about 50 per 10 minutes. It’s the one that is a large room on top of a thin tube with water pushing mobs into it, so they fall down. Also I use the actual one on my 1.13.2 world. The mob spawn is broken. If I quit and rejoin, mobs spawn well for 3 minutes then stop. I am on singleplayer

This seems to happen in the last few snapshots as well, at least including 19w02a and 19w03a, maybe some earlier ones. Should perhaps reopen the ticket.

Yup mob stopped spawning on 19w02a

Is there a consistent method used to track where defects are ported/fixed in other versions? For example, is there a way to see which corresponding defect for Bedrock that this would be ported to, assuming that it is also an issue there?

seems to be a problem again in 1.14