https://github.com/BlueMap-Minecraft/BlueMap/blob/bleeding/BlueMapCore/src/main/java/de/bluecolored/bluemap/core/resourcepack/BlockStateResource.java#L347

The way we merge the variants there, we are always iterating over an empty list, so there is no possible way this list could ever populate.
To fix this, we need to just copy the propertyVariants if the oldVariants list is empty.