I updated my map from version 1.14.2 to 1.14.4, playing on server, and since then no more hostile mobs are born. I don't know if it has anything to do, but I don't think foxes either.
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What is /gamerule doMobSpawning
set to?

The /gamerule doMobSpawning is true
Please attach the affected (zipped) world file to this ticket. If the file size is too large, please upload it somewhere else and then link to it here.

Any news?
I took a while to check out your world, and it seems like there are mobs on the edges of the loaded world that don't despawn, but take up a significant part of the mob cap. Once they are killed (or despawned), all mobs spawn without any issues again. I found that we already have another ticket about this though, so I'll resolve this ticket as a duplicate of it (MC-155289). Feel free to add a vote to it though!