I don't think Mew should be spawning in an ocean biome. The wiki still tells us he is supposed to spawn in Jungle, but in the 4.3.0 and 4.3.1 versions he spawns in oceans instead. Moreover, I waited over 10 minecraft days for Regice to spawn but he didn't. I'm not sure if this is a bug as well or just bad luck. Anyway, I think there is something wrong with some legendary spawns.

Are you using Spawn of Pysduck? This can be caused by a bug that was already reported. Still, you will need to give information of your setup, because Mew spawns, and any other legend for that matter, cannot be modified unless done in purpose (SoP, spawners, command, etc).

As for Regice, legendary spawns are rare by definition. Try turning up your spawn settings in the pixelmon.hocon, see if that makes it spawn better.

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