in the minecraft 1.13 pre2 update there was written:
MC-129620 - Mob spawning algorithm is super slow
This has fixed my mob spawning in the overworld and nether but my end dimension is still bugged. The mob counter in F3 shows that there are 0 mobs around me. It goes normal once i enter the overworld or nether but stays at 0 in the end.
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What version was this world created in? Also, what version(s) have you upgraded the world to?

Made in 1.12.2
then upgraded to every single version of snapshot from 18w01a to 1.13-pre2