Only vanilla mobs, player mobs, and trumpet skeletons seem to be spawning at my meadow biome base. Only by going to the next biome over do I see karate zombies etc.
Also, since the 3.1.0 update skeletons have been spawning without bows and just attack me melee despite me being in age 5. Even the skeletons in twilight forest, which do spawn from spawners with bows, just prefer to wack me instead of shooting.
Meadows should spawn all the hostile mobs of similar biomes, and skeletons should use their bows at the proper age.
Mobs are gated behind the different ages. With each new age you unlock more mobs.
Following on from what SuperSandro2000 said, run /gamestage info and check what stage you have that ends _mob. If it's not five_mob remove it with /gamestage remove @p <stage_name> and add /gamestage add @p five_mob
It says creative_mob. I did recently unlock the creative age, now that I remember.
The issues with the mob spawning only seem to be in the meadow biome. It might be happening elsewhere too, but my base is in a meadow, so it's most noticeable. This has actually been happening even before the 3.1.0 update, and mobs are spawning correctly in the nearby biomes and the hunting dimension, minus the skeletons.
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Following on from what SuperSandro2000 said, run
/gamestage infoand check what stage you have that ends_mob. If it's notfive_mobremove it with/gamestage remove @p <stage_name>and add/gamestage add @p five_mob