I think I've figured out what could be the cause:

Terrestria uses the base library Terraform, which has a class for defining small log blocks (such as sakura logs). This class creates its own WATERLOGGED property:

https://github.com/TerraformersMC/Terraform/blob/844a986daf93e908202d823cc6f733d5909a861d/terraform-wood-api-v1/src/main/java/com/terraformersmc/terraform/wood/block/BareSmallLogBlock.java#L50

	public static final BooleanProperty WATERLOGGED = BooleanProperty.of("waterlogged");

Thus, if you do state.get(Properties.WATERLOGGED), you will get into a situation where there exists a waterlogged property in the state where waterlogged != Properties.WATERLOGGED but waterlogged.equals(Properties.WATERLOGGED) == true.

HydrogenImmutableReferenceHashMap only uses reference equality (==), not the equals method (.equals()), so it will fail to find this special waterlogged property, but in vanilla, the equals method is used, so it works.

The question then becomes: whose fault is this? Clearly, Terrestria / Terraform should just use Properties.WATERLOGGED if they are going to be accessing the property through Properties.WATERLOGGED instead of creating their own property.

However, given that this occurs at the very least with a few mods, Hydrogen has a few options:

1. Use .equals() for a slight performance hit. Since most implementations of .equals() check reference equality first, this shouldn't cause too many problems.
2. Warn when mods try to construct properties that are equivalent to existing properties through .equals() but not through reference equality
3. Do nothing and wait for those mods to fix the problems on their side.

It looks like BetterEnd will be fixing this in a future release: paulevsGitch/BetterEnd@67a4b19

As well as BetterNether: paulevsGitch/BetterNether@40ce83e

I will try to ship a fix for the Terraformers' mods soon.

This has been fixed on Terrestria's side in Terrestria 2.2.0, however BetterEnd and BetterNether do not have fixed versions yet.

It might be possible to "intern" or cache calls to BooleanProperty.of like String.intern() does, so separate calls to BooleanProperty.of(x) return the same object. That would guarantee the uniqueness of Properties at all times, and allow reference equality to be used without extra care taken by other mods.

Sandwichable also has a similar issue.

https://paste.ee/p/HhwLq

Sandwichable also has a similar issue.

https://paste.ee/p/HhwLq

I just tested with #22 and it seems to fix it.