From developer confirmation, the Creaking Heart currently checks if a dimension's fixed_time is set or unset to determine whether or not a Creaking should spawn. This works fine for vanilla's dimensions, but some modded dimensions like Twilight Forest use fixed_time and don't feel so unnatural to prevent a Creaking from spawning.

According to the Minecraft Wiki, Mojang's implementation of this feature uses the natural tag instead of fixed_time. Changing this behavior would provide more parity with vanilla and allow for more uses of the Creaking outside of the Overworld.

Thanks! I'll take a look on it if I have time today.

Okay, I just checked and the mob does look for the natural value from the dimension to spawn, I added a RequireNaturalDimension config to disable it if needed. It also checks if its night in the dimension they're in, however the vanilla way to detect if its night on the dimension is:
(From the Level class)

public boolean isDay() {
    return !this.dimensionType().hasFixedTime() && this.skyDarken < 4;
  }

  public boolean isNight() {
    return !this.dimensionType().hasFixedTime() && !this.isDay();
  }

It checks if the dimension isn't a fixed time dimension (two times lol) and check if the sky is dark enough. I'll simplify this on the mod to val isNight = level.skyDarken >= 4 to make it possible in other dimensions but note that vanilla (1.21.4) won't be doing that as it is always checking for the fixed time.

As an extra I also made Creakings with name tags not despawn when its day, but it still despawns with distance from the heart. And on the previous bug fix update I fixed the swim behavior from the Creaking. This should make telepathic redstone more viable, but also note that on vanilla it won't be like this 😅

@sm644me now I can confirm the Creaking is spawning in the Twilight forest, would it solve the issue for you? If yes, I will publish a new version with the updates (v1.2.1) so you can test, and if there is another thing to fix I can reopen the issue!

@danbrown That would solve it! Strange that the vanilla behavior is like that, though...So I'm sorry for the issue being somewhat incorrect. But anyway, this would work very nicely and probably won't lead to future issues! Thank you!

@sm644me version published! I think Modrinth already approved it. I hope you can use it well! Consider joining my discord, I have other mod projects and some help with suggestions are always welcome!